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The sponges—Their significance
THE SPONGES--THEIR SIGNIFICANCE* (Conclusion) From Aprds Darwin, by DR. HI~LAN JAWORSKI
The sponge gives its structure to the lymphatic system. In bone, the part played by sponges is very important, and in certain of the cranial bones it predominates. The basic animals, polyps and sponges, represent the primitive materials of all our organs. ROLLED-UP Oll itself like a ball of string, the glomerulus of the kidney resembles a spiral thread which at the slightest shock uncoils and spills out its poison. This thread is the essential element of the sting cell. In the kidney, it ejects urine. "Amphioxus possesses two excretory organs which are both analogous to those of invertebrates . . . I n the light of blastodermie homology, the kidney of Amphioxus should be compared to the sudorific glands. ''12 The sudorific glands! But there does not seem to be any possible anatomical connection between the glands which produce sweat and the renal glomerulus; the sudorific glomerulus is glandular, whilst the renal glomerulus is vascular. None the less, from the physiological point of view, the production of sweat is a complement and an adjuvant to the secretion of urine Furthermore, the glomerular process in the kidney is not a simple act of mechanical filtration, but one of elective filtration. To understand how intense is the secretion of urinary poison, consider that in just over two days a man produces enough urine with which to kill himselfi The toxicity of sweat is also very high. Man has about two million sudorific glands. Let us take another look at sting cells, to see whether the analogy with the kidney and the sweat glands becomes any closer. Perhaps at the same time we shall come to understand why there are no sting cells in sponges. We know t h a t the tegument of polyps is distinguished by the presence of innumerable sting cells, sometimes grouped into batteries. We also know that they can be interiorized, into the stomach for instance, where they live on the food that is swallowed. ~ Organs of attack, of defence, and of nutrition, the sting cells are sensitive and glandular. The bell of a medusa (Gemellaria Implexa) shows on its external surface four pear-shaped pockets full of these sting cells. Here, their presence is difficult to explain; they probably play a part analogous to those poisonous rods which have been demonstrated in certain worms (Tubellaria) and which biologists call "rhabdites". Today these rods are recognized as having the same nature as sting cells. Sting cells can be found interiorized in Solenaucolons (polyps), and Edmond Perrier says t h a t they act to defend the animal against internal parasites. But it is above all in the siphonophores, particularly Disconanthes, that the interiorization of numerous sting cells within the tissues of the animal deserves special notice. *Translated by Dr. M. ttarling, and published by the kind permission of the New Atlantis Foundation. 279
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Between the vault of the gastric cavity and the base of the float of these siphonophores, canals can be seen amongst a colony of sting cells. When the canals leave the gastric cavity for the walls of the float, they cross the peripheral parts of the sting cell colony, and send branches into it which ramify into a network. This network "is considered to be a tubular gland connected with the secretion of digestive juice, and the combined structure has been called the
liver".14 These cells do indeed contain brown granulations which give the whole mass a dark colour, visible through the teguments. The lower canals are different, and their cells contain, not brown granules, but guanine crystals. "This part is thought to be a kidney, but one without special excretory channels." Granted t h a t the mode of transformation of these interiorizing sting cells is unknown, their connections with gastric function and these precursors of the liver and kidney are truly interesting. A certain number of siphonophores eject a coloured excretory fluid which tints the water around them. This secretion is produced b y certain cells of the shield which are basically transformed sting cells, loaded with pigment. Already, amongst the Protozoa, Cannopylaria demonstrate a curious glandular organ, the "Ph~eodium". I t is a pigmented mass whose function is unfortunately still unknown, but in which large cells can be observed which are "distinguishable b y a spirally crossed striation which recalls t h a t of certain nematocysts (sting cells) before they burst". So, far from disappearing, the theme expands . . . The phmodium bears a striking resemblance to the female breast. I n this connection, it should be recalled t h a t the milk-producing gland is no more t h a n a specialized sweat gland. If, behind the complexity of organs (it always comes back to this) we look for function, then i t is evident t h a t the same fundamental theme is being played, here exteriorized, there interiorized, with variations in the different organs which can be shown to have a common source. Thus a study of the sting cell not only shows the connection between the vascular renal glomerulus and the glomerulus of the sudorific gland, and even of the m a m m a r y gland, but also connects the functions of these structures with the function of the liver. B y now it will come quite naturally to us to see t h a t bile is also a product of elimination, even more toxic t h a n urine, to establish t h a t it plays an import a n t role in digestion, to know t h a t it has antiseptic properties, t h a t it disinfects the intestine, and kills the germs which would otherwise invade the biliary passages. Has not Ed. Perrier attributed this latter function to the intcriorized sting cells which defend the animal organism against internal parasites? The transition of an interiorizcd sting cell shows us why it is not found in glands, which are spongy organs, any more t h a n in the sponges themselves. The reason is t h a t '%he sting cell is at once a glandular element and a transformed vibratile cell whose product of secretion is a mucoid and venomous substance".15 I n the transformation caused b y interiorization the vibratilc defence element disappears, whilst the glandular, secretory element develops. I f we examine a drop of liquid from the cavity of the sponge, a cell having amoeboid movement is seen. I t is our old friend the Amoeba, the white cell, if you like, which will appear again in the internal fluid of the echinoderms, and m the blood of the shrimp, the toad, and man. " I f into the space beneath the skin of the back, t h a t is, into the dorsal lymphatic sac of the toad, is injected fluid containing powdered carmine in suspension, and then a fragment of elder-pith is introduced into the same lymphatic sac, it will be found some time later t h a t the cellular cavities of the
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elder-pith are filled with lymphatic cells and that the latter are full of carmine granules. These cells, being very active amcebocytes, have ingested the carmine suspended in the lymph (phagocytosis), and then penetrated into the elder-pith (migration).'16 In higher animals, only the cells derived from embryonic jelly have the property of phagocytosis. In polyps, the cells of the endoderm have it, but amongst the sponges all cells, those of the tegument as well as those of the internal lining, possess this property. We always find it in the cells of the vessel walls in vertebrates, in those lining the renal tubules of annelids, ete . . . . This property is important, for it gives a special character to those cells which possess it. Thus white cells consume all residues, dust, etc. In leeches, the white cells accumulate beneath the skin around foreign bodies, forming a crusty layer. They join battle with microbes, and their bodies form the pus. The earthworms contain white cells which, suspended in the general fluid, are regular little mobile glands filled with globules of unknown nature. The white cells of echinoderms and of bryozoans are floating excretory cells, in the same way as those of certain worms constitute true unicellular glands. In the walls of the intestine the white cells dig themselves holes in which to live, and which are the "blind pits" which function as glands of the intestine. "So", says Prenant, "glandular cells can be seen, now fixed, forming part of solid tissue, now mobile, swimming in their fluid or moving freely throughout the organism; the latter case is that of the red cells of the blood and of the phagocytes." On the other hand, apart from the internal lining and from the capsule-the rind of the sponge--apart from the skeleton, in fact, the substance of the sponge itself is analogous to the interstitial substance of connective tissue. This amorphous substance shows every degree of transparency and of consistency, from nearly fluid jelly to cartilage. Cradled in this jelly, as we have just said, are migrant undifferentiated cells which are identical with amcebse, and with white cells. Now, these migrant cells of the sponge, these interiorized amceb~e, potentially contain the whole sponge, since all parts of the sponge derive from their differentiation, the gland cells of the tegument, and those which produce the skeleton and the sexual elements. These amcebm of the sponge sometimes also assemble in large numbers in certain parts of the body, notably in damaged parts. I f its embryonic origin was of absolute value in demonstrating the significance of an organ or of organisms, this fact alone would reveal the glandular theme of the sponge, according to the principles established in the preceding chapter. But the embryogenic approach is too wide, too large, and leads too quickly to unification. The white cell must be regarded from another point of view to see whether the glandular theme of the sponge can be confirmed. White cells are formed in the lymphatic glands. There, whilst still free lymphatic cells, they can scarcely be distinguished from the fixed cells. Moreover, these lymphatic glands themselves are clusters of elementary nodules formed in the first place by the accumulation of white cells. This formation is complicated by the budding of a lymphatic vessel. " I f this process takes place at the same point in a group of lymphatic vessels running side by side in this or that region, there will be seen to form simultaneously several lymphatic nodes which, through peripheral fusion, constitute the complex lymphatic gland of man. ''17 Thus, the white cell leads us to the lymphatic node, and we see that the complex gland is formed from elements, as is the composite sponge. Parallelism goes farther; the regular lymphatic channels become, in the
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glands of birds, more and more complicated. They become irregular, tortuous, and cavernous in the lymphatic glands of mammals. Finally, the tissue surrounding the gland becomes fibrous, constituting a rind, the capsule. And this is not restricted to the lymphatic glands. "Certain lymphoid organs are formed in the middle of embryonic connective tissue, by concentration of white cells at certain points, and through transformation of the connective tissue into a network. Thus can be roughly represented the formation of a lymphatic gland." Of what use, it could be asked, are the complicated cavities of the gland, any more than those of the sponge. The reasons are identical. In both cases this complication exists to delay the flow of liquid, and to increase the active surface through the production of crypts. I n the lymphatic gland processes can be traced, issuing from the capsule and forming a series of links, which exactly correspond to the sub-cortical sinus of the sponge. The central substance of the lymphatic gland is made of the same tissue as the sinuses; only the links are finer and narrower, and crowded with white cells. I t is at this stage that the gland produces certain of the cells of the blood. I n a sponge all the solid bodies contained in the substance--spicules, algae, etc., are wrapped in a thick coat of connective tissue cells. These cells are very numerous around the reproductive elements, and are, moreover, more tightly packed the nearer they are to the object which they are enveloping. The reproductive elements are constantly developing to the detriment of the amoeboid cells. Those parts which, in the sponge, give rise to the elements of reproduction are differentiated, in the organism, to supply the living elements of the bloodstream.
To recapitulate, allowing for variations in detail, the fundamental structure of the sponge is the same as that of the lymphatic gland. On the other hand, the gland is broached, on its convex surface, by numerous lymphatic vessels, "afferent vessels", which go through the capsule and open into the cortical sinus, and whose walls are continuous with the spongy tissue. Here we are reminded of the system of inhalatory canals of the sponge, which in the sponge of the outside world stop short at the mouth of the inhalatory pore. From the concave surface of the sponge run one or a small number of trunks, the efferent vessels, which correspond to the exhalatory system of the sponges; only, in the spollge, these canals terminate in the same way at the cloacal orifice, whilst in the gland they are prolonged into the lymphatic vessels. I n short, lymph arises at the glands by means of afferent vessels supplied with valves, and enters through the convex surface; it is contained in the large limks of the sub-cortical sinus, and then in the small canicular spaces which separate the more solid masses; it flows around these latter and drains away in all directions through the cavernous channels into the terminal sinuses which we can call "cloacal sinuses". Finally it reaches the excretory orifice on the concave surface and leaves through efferent vessels which are larger and fewer in number than the afferent ones. The basic laws of the sponge can be seen here. Parallelism with the structure of the sponge is, it seems, easy to trace. Just as those nodes also called lymphatic glands are really no more than swellings on the course of lymphatic trunks, this study now leads us quite naturally to that of the lymphatic vascular system as a whole. In a moment we shall have to consider the spleen, which is nothing more than an enormous lymphatic node, and we shall be in a better position to see whether the spongy themes we have uncovered still persist.
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In embryos, lymphatic vessels, as buds from the veins, appear before the lymph nodes. "Thus is verified Ranvier's proposition, t h a t the lymphatic system develops like a gland, an offshoot of the tree of the venous circulation. ''is The lumen of lymphatic vessels is generally fairly wide. When they meet the confluence is much dilated; they even form veritable lacunae in the frog, for instance; and, in connective tissue, canals are formed by lymphatic spaces. Here again we have a renal apparatus, not merely passive, but secretory. The lymphatic canals alone constitute the lymphatic system in fishes and batrachians, which have no lymph nodes. What then is the significance of canals in this case? Professor Prenant will tell us: "The lymphatic system of batrachians, which has become well known through the work of Rasconi and of Panizza, and later through that of Ranvier, is developed in such a way that it can be said of the frog that it is a lymphatic s p o n g e . . . " Here it might be thought that the word sponge is being used in an allusive sense; but this would be an error, for the parallelism is fully justified. In fact, "there exist in these animals capillary lymphatic vessels, fixed, or dilated into lakes in the skin and in different organs, where they form networks (Langer, Ranvier). These original networks are drained by trunk channels, often very short, which open into vast, incompletely partitioned sacs, 'sinuses' or 'lymphatic reservoirs', which are results of the fusion of ordinary capillaries which originally were distinct from one another (Ranvier)." "Thus the capillary networks of the skin communicate by means of very short capillaries with the large sacs, 'subcutaneous sacs', which lie immediately below the tegument. The intestinal lymphatic networks open in the same way into a vast lymphatic reservoir called the 'retroperitoneal cistern', situated under the peritoneum in front of the vertebral column; communication is established by means of capillaries which join one another around the mesenteric arteries, forming 'perivascular lymphatic sheaths'. These great lymphatic sacs had previously been taken for serous spaces not bounded by their own walls. Impregnation with silver has revealed otherwise, that they are bounded by lymphatic endothelium, and a study of their development allows us to hold that they are derived from fused lymphatic vessels. ''19 I t could be said that whereas in the sponge its canals end sharply at their orifices, the fact that lymphatic canals are prolonged into vessels establishes a radical difference. But, as Prenant tells us, if from the morphological point of view the lymphatic system is contained in closed vessels with visible cellular walls, then from the physiological point of view the system of intercellular lacunm of the tissues must be added to this system of canals in order to conceive of the lymphatic system as a whole. The one is a system of preformed channelled routes, the lymphatic routes proper; the other is but a mass of lymph channels lacking both boundaries and permanent existence. Together they represent the complete structure of the sponge. Claude Bernard has said that the anatomical elements of our tissues live in lymph as fish live in water. Indeed, if the blood vascular system is a mechanism of irrigation, that lymphatic system is one of drainage. Its function is to remove from our tissues substances deriving from the blood which has bathed and nourished these same tissues. All this now leads us to consider the serous cavities. The serous cavity is part of the general body cavity in the form of a closed bag (contained between the two layers, parietal and visceral, of the mesoderm). We know that there are three compartments, two thoracic and one abdominal. An extension of the latter is called the vaginal cavity, and contains the testicles.
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The serous membranes, which can be called "lymphatic layers", are all more or less alike, and to understand them the whole peritoneum can be taken as an example. Since their disposition is a m a t t e r of adaptation, it is known t o d a y t h a t their morphology is due to the movement of the organs they surround, rubbing, sliding, e t c . . . At first sight there is nothing about their characteristic formation to suggest the sponges. But none the less, ff only a little trouble be taken, it is not difficult to find, in the midst of detailed modifications, the basic original type. Amongst certain primitive animal species the peritoneum is often ciliated; these cilia m a y also appear in higher forms, in the batrachians for instance, where they guide the egg which has fallen into the peritoneal cavity. As the serous membrane is a sponge, it can contain muscles. I n fact, we do find sub.peritoneal smooth muscle of the broad ligaments, the round ligaments, the superficial part of the uterine musculature... Let us consider the peritoneum in one of its important folds, the great epiploon. I f this fold was originally spongy, how can we trace the complicated system of the canals of the sponge in this membrane ? Will it perhaps be difficult ? B u t nature makes canals very easily. At first the epiploon is a thin membrane formed of two layers of cells; then, there develops between the two a median layer of connective tissue. Later, in the second year (in rabbits), holes appear in the epiploon which, blind at first, later become perforated. In this thin membrane, the holes represent the canals of the sponge. I n m a n the definitive appearance of the epiploon is that of fine lace often with wide interstices due to the passage of white cells. This membrane is extremely mobile; its active movements are not of muscular origin, but are perhaps due to the very large n u m b e r of white cells which it contains. The great epiploon carries to an extreme the scavenging role of its amcebse; thus the abdominal cavity is kept clean. There are in it lymphatic nodules (milky patches) and blood-forming elements which cause it to rate as a veritable gland. " F r o m this it can be concluded t h a t the great cpiploon is, like the spleen, a h~ematopoietic organ; this function could be explained b y the common origin of this membrane and of the spleen (Pardi). ''2o The spleen itself is composed of a fibrous envelope or capsule which sends processes or trabeculse into the interior of the organ. Thus, the spleen is divided into segments. From these trabeculse there arise at certain points other secondary trabeculm which subdivide the segments or lobules into numerous element a r y lobules, in such a way t h a t the organ can be regarded as composed of 500 million elementary lobules. I n the spleen, as in all other organs, blood arrives through the artery and leaves through the vein, thus clearly constituting an afferent or inhalatory system coupled with an efferent or exhalatory one. The walls of the artery gradually fuse with the splenic pulp itself. I n this pulp, which consists chiefly of an infiltration of white cells, bodies are found which formed from thickened tissue filled with the same white cells. The canals lead into areolae which in turn are continued into the enormous exhalatory system of venous canals, where highly contractile "rod" cells are found. I t is through their contraction t h a t blood is driven from the venous sinuses into the veins, a mechanism identical with t h a t which causes the circulation of water in the sponge. The immense venous network of these canals forms a major p a r t of the splenic pulp. From the confluence of the venous sinuses arise little veins which join to form larger trunks which in their turn finally unite to produce the efferent vein. W h a t must be clearly understood is t h a t the arteries and veins of the spleen
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simply represent the bloodstream's routes of arrival and departure in an autonomous organ, whose structure is really that of a dense sponge, and whose active surface is therefore enornous. The more complex functions of the spleen are unfortunately not completely known. This organ is essentially a centre for the destruction of old red cells (debris), and for the production of white cells (amoebae); it has a secretory function, and accumulates carbon, microbes and pigments; it is also a storage place for iron (fixation of colours). L y m p h nodes develop slowly after birth, as the growth of sponges in general is slow. Speaking of lymph nodes, it should be noted that if these tighly-packed masses sometimes appear to have a uniform consistency, more often they show a clear centre, called the "germinative centre", which has been held to be the source of all the different sorts of cells. Now, there is a close analogy between, on the one hand, the formation of white cells and genital cells in the organism, and on the other, between the reproductive and amoeboid elements of the adult sponge. The reproductive elements are generally formed in the genital glands. Now, in polychete worms and nemertids, the jelly covering the peritoneum can produce eggs or spermatozoa from any point on its surface. In the whole class of Gephyrea, the genital organs are but an appendage of the peritoneal membrane, and in some of them the genital elements develop in the peritoneal cavity (general body cavity). Now one can understand why, the peritoneum being fundamentally a sponge, the sexual elements derive from it; the differentiation of sexual glands is but secondary. A fusion can already be seen in the oligochete worms, and in Rana temporaria it is still more obvious that genital cells are produced by progressive differentiation of peritoneal cells. Epithelial cells of the peritoneal membrane can even form placental elements, a fact which has greatly surprised certain authors, though it is really very natural, since the placenta is a spongy organ. The spermatozoa of the sponge appear to develop constantly at the expense of migratory amoeboid ceils. In most of them these amoebae subsequently divide into spermatids which remain grouped; the same amoebze also give rise to female elements which produce eggs. They also remain together. Fertilization of the eggs always takes place inside the sponge, and the e m b r y o develops, either in the canals, or even in the substance of the animal itself. Altogether, it follows from this that white cells in higher animals are no longer able to differentiate into reproductive elements, this function having been taken over by the sexual elements where, as we have seen, atavistic notes sometimes appear, recalling th~ primitive spongy theme. In other words, leaving out the questions of adaptation and homogeneity, the only difference which exists between the sponge and the spleen, for example, is that amoeba~ in the spleen do not transform into reproductive elements. This is a simple question of division of labour which, moreover, has come into operation gradually, as Rana temporaria shows us. Perhaps one could say that blood establishes a major difference between plant.animals and our organs. But one only has to follow its phases in the organism to be aware of the clumsiness of such a concept. Blood leads to lymph, lymph to simple serous fluid, and that by a change in the position of the organ. Serous fluid is like sea-water, and yet in the ox, the peritoneal serous fluid contains more white cells than the blood. Finally, it must be noted that regeneration is as difficult in sponges as it is in glandular organs in general. It is very important to maintain the parallelism which relates the gland,
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the sponge interiorized within an organism, however differentiated and transformed, to the spongy species in nature. We have seen t h a t the internal cavity of sponges is gastric, secretory, excretory, and circulatory. I t is also reproductive, and we could add respiratory, although there is as yet no localized or differentiated respiratory apparatus in sponges. All the functions, in fact, are found there, and it would be erroneous to exclude a n y of them. We cannot close this chapter without taking a look at the skeletal structure of the sponges. There are sponges with calcareous skeletons, sponges with siliceous or horny skeletons, and soft sponges without any skeleton at all. The skeleton is formed b y skeletal needles or spicules. I t is difficult to draw a line of demarcation between the spongy species which we have named. The calcareous spicules are joined b y horny substance,.and correspond up to a point to the collagen fibres and elastic fibres of vertebrates, of which, indeed, they are a simplified form. I t is known, in fact, t h a t structural modifications characterize gelatinous, collagen, and elastic tissues. Spicules can be single, composite, or fused; there are also spicules in striated lamell~e. Others have the form of scales, are mobile, can transform into defensive spines, or on the other hand can be superimposed like the tiles of a roof to act as a protective covering. Certain protozoa already possess analogous structures, but here, according to the species, they become of great importance~ combining wide variation with remarkable stability. Spicules are formed in large granular cells, the "spongioblasts", which in the course of their development turn more or less completely into spicules. In horny sponges the skeleton is made of a substance called "spongin" which bears a certain analogy with silk. Certain tropical sponges have been found to contain iodine in organic combination, giving the substance "iodospongin". To sum up, the spongin which forms the framework of the trabeculse of the sponge is achieved by total or partial transformation of the special cells called "spongioblasts". Now, the framework of the spleen is also formed by the transformation of large granular ceils into a special substance which performs exactly t h e same function as spongin, and which has been called "reticulin". The process of this transformation has been observed in selaeaous fish; and it seems to be almost identical in sponges and in the spleen. Further, in the course of formation of the sponge's skeleton, there appears to be a destruction of spongin performed by special ceils, the "spongioclasts", analogous to "osteoclasts", according to Delage. We shall return to this later. I n a Malayan sponge (Semperella) the skeleton is formed b y a layer of silica uniting the spicules into a rigid grille. This phenomenon, seen in most siliceous sponges, has caused Deluge to say: "This growth could not be better compared than with t h a t of the diaphysis of a long bone, in thickness to the periosteum, and in length to the epiphyseal cartilage, without intercalary growth. ''21 I n Jenthella, the network formed b y the fibres presents s o characteristic a structure t h a t Deluge cannot resist writing, in spite of his horror at the analogy: "The structure of the external layers recalls t h a t of cartilage, except for the regularly concentric arrangement of the cellular elements. ''s2 From another angle, in membranous bone such as the cranial bones, for instance, ossification starts b y the formation of veritable osseous needles, a stage which we can call: spiculous stage. One end of the needle joins others to form the central plaques, whilst the other extremity accounts for the serrate edges of the cranial bones.
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In man, though bony tissue m a y have been laid down during a formative period, it is still a long way from completion. I t s shape and internal structure are due to two contrary processes. I t is, says Duval, as if two architects were working, the one undoing all t h a t the other had done. I n the midst of this apparent chaos, this internal strife, the definitive shape of the bone gradually appears, thanks to a "sculptural resorption". Will this new phenomenon be an exception to the parallelism between spongy and bony structures? The young, fibrous, thick bone is always dense, that is to say, it has no medullary canal. The marrow, a foreign substance with a structure all its own, is destined to find its place within the medullary canal. So who is the architect who will dig this canal? Let us study the sponges to find the answer. Oysters sometimes carry special marks, a sign of what has been called "spicebread disease". This peculiarity, which does not seem to be very harmful to oysters, is due to the presence of a curious sponge, Cliona. This sponge usually lives inside calcareous objects, such as pebbles and the shells of molluscs; externally it only shows as small points corresponding with the orifices which communicate with the outside world. This is what Delage says: "The interior part forms a network, contained in the galleries which the sponge hollows out in its host. This is the reticulate form. I n certain cases, where the host has become riddled throughout its length, the sponge, in order to go on growing, overlaps the end and turns back on itself. I n the end it m a y undergo such considerable development t h a t the host, engulfed, disappears; it then takes a massive form. ''23 I n the light of interiorization does not this description suggest the process of bone formation? especially if the last stages are considered as exaggerations which are found in the organism under pathological conditions? But this is not all. Just in the same way t h a t bone marrow, though it is spongy tissue, is not typical of glands in general, so this sponge seems to be an exception to the basic law. The relatively very large canals are only indifferently exhalatory or inhalatory. Moreover, Clionas, of which there are m a n y varieties, are not the only perforating sponges. There are others which have a siliceous skeleton, whilst Oscarella, for instance, has no skeleton at all. T o sum up, perforating sponges develop the osteoclastic theme which has already been noticed, and represent the destructive architect who is indispensable to bone formation. These thoughts on bone force us to recall the polyps, amongst which we have already found spicules and osseous notes. Bone, therefore, represents a combination of polyp and spongy themes. Now, in this combination, the perforating sponge at least, if not all of the sponges, stands, as we have seen, for that other architect of whom Professor D u r a l speaks, he who is in constant conflict with the first, constructive architect. Will this first architect be found in the camp of the polyps? I f we are right, we shall find opposition between sponges and polyps. I n fact, we are saying t h a t "contrary to the universal rule for sponges, the ciliated ectoderm is invaginated into the endoderm (in the transformation of the gastrula), so t h a t in the adult the normal relationships of the layers are reversed ~' .24 I n other words, the sponge, from certain points of view, is like a glove turned inside out, a polyp back to front, whose epidermis has become its internal lining, and whose collar cells derive from the transformation of tegumentary cells. Whether this reversal of invagination in the larvm of the ancestors of sponges
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was due to mechanical causes or not does not m a t t e r much. I t is only necessary to remember the existence of this opposition which manifests both in the absence of regeneration, and in the completely internal activity of the sponge. Although t h e y have no eyes, the larvm of siliceous sponges, sensitive to light, seek darkness, whilst the green hydra seeks light for its algae, just as the shoot of a tree seeks it for itself. Polyps extending their agile tentacles, medusm scudding on the waves, both show an external activity. This opposition which has been pointed out b y Y. Delage, is not absolute; on the contrary, more often the two forms complement one another. Let us look at them again, in the organism and in nature. The blood vascular apparatus merges imperceptibly into the lymphatic system. I n batrachians and reptiles, as in certain fishes and birds, there are, in the course of the lymphatic current, organs endowed, as is the heart, with rhythmical contractions; they are called "lymphatic hearts". Thus, here also we find fused notes. I n the course of vessels are found spongy organs; spleen, lymph nodes; so t h a t it is natural to find inside the bones themselves marrow, a spongy substance. These complicated fused notes cannot be attributed exclusively to polyps or to sponges; really they belong to both. Classification exists principally in our minds.
Nonetheless, it must be recognized that polypian characteristics predominate in vesse/s, spongy characteristics in glands. This is what we wanted to establish. At the same time, this is how the internal reality of the organism can be approached, the reality of an incessant activity which is killed by immobilizing words. I t is, in fact, an association, a joining, a complication, a perpetual process of mutual help as well as a constant struggle. Red cells originate in the organs which will destroy them, bone is not only the result of the battles which we have indicated, it owes its shape to opposing pulls. Cartilage has already had to disappear to make way for it, and the point of ossification, primitively the point of calcification, sees its walls founder where the vessel penetrates. Polyps and sponges thus seem to be interiorized within the organism like fundamental animals, that is to say, representing the primitive materials of all our organs. Their themes become more marked in pathological phenomena. Think only of the gelatinous plaques which, developing on arteries, become infiltrated with calcium, of the calculi of all sorts which appear in the salivary glands and in the kidneys, and it will be seen t h a t if the deposition of calcium is normally restricted to special areas, primitive tendencies and atavistic memories can awaken out of time, and by sharp pain remind us of their original character. Up till now we have considered polyps and sponges as opposed to one another. But this is only relative, and there can also be strong sympathies between these animal groups. The deep sea fauna includes creatures upon whose classification authors are not agreed. Those which are held b y some to be sponges, are for others gigantic colonies of protozoans, or again polyps living in symbiosis with sponges. All authors, and this is very important, have noted the similarity between sponges and coral polyps, osteoid polyps we should say, since this group shows the chief osseous note. Hyalonema is a sponge shaped like the calyx of a flower, borne on a stalk 30 centimetres high. On this stalk a polyp becomes fixed, so constantly t h a t it was thought to be the skeleton. More and more new cases of symbiosis are found . . . The opposition between polyp and sponge is thus quite relative. Almost always we find them side b y side in nature, often exchanging functions. We
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can go further. Polyp and sponge have a tendency to unite, even whilst o p p o s i t i o n e x i s t s b e t w e e n t h e m , so t h a t w e c a n b e s t e x p r e s s t h e i r r e c i p r o c a l r e l a t i o n s h i p s as r e p e a t i n g , : i n a n o t h e r m e d i u m , t h e t w o g r e a t m a n i f e s t a t i o n s of the vegetable world: polyp, the stalk; sponge, the root. REFERENCES
12 Delage, Yves, et Herouard, Edgard, Traitd de Zoologie concrete, Vol. VIII, p. 313. Sehleicher freres, Paris, 1899. la Jaworski, H., Pourquoi la Mort? pp. 48, 73, 224. 14 Delage, Yves, Op. cir., Vol. II, p. 256. 1~ Prenant, A., Bouin, P., et Maillard, L., Traitd d'Histologie, Vol. I, p. 178. Masson et Cie, Paris, 1911. le Prenant, A., Op. cir., Vol. I, pp. 543, 544. 1~ Branca, J., Prdcis d'Histologie, p. 327. Bailli~re et ills, Paris, 1914. is Prenant, A., Op. cit., Vol. II, p. 118. 1D Prenant, A., Op. cir., Vol. II, p. 122. 20 Prenant, A., Op. cir., Vol. II, p. 305. ~1 Delage, Yves, Op. cit., Vol. II, p. 131. ~2 Delage, Yves, Op. cit., Vol. II, p. 142. 2a Delage, Yves, Op. cit., Vol. II, p. 173. 2a Delage, Yves, Op. cir., Vol. II, p. 61.
The sponge gives its structure to the lymphatic system. In bone, the part played by sponges is very important, and in certain of the cranial bones it predominates. The basic animals, polyps and sponges, represent the primitive materials of all our organs. ROLLED-UP Oll itself like a ball of string, the glomerulus of the kidney resembles a spiral thread which at the slightest shock uncoils and spills out its poison. This thread is the essential element of the sting cell. In the kidney, it ejects urine. "Amphioxus possesses two excretory organs which are both analogous to those of invertebrates . . . I n the light of blastodermie homology, the kidney of Amphioxus should be compared to the sudorific glands. ''12 The sudorific glands! But there does not seem to be any possible anatomical connection between the glands which produce sweat and the renal glomerulus; the sudorific glomerulus is glandular, whilst the renal glomerulus is vascular. None the less, from the physiological point of view, the production of sweat is a complement and an adjuvant to the secretion of urine Furthermore, the glomerular process in the kidney is not a simple act of mechanical filtration, but one of elective filtration. To understand how intense is the secretion of urinary poison, consider that in just over two days a man produces enough urine with which to kill himselfi The toxicity of sweat is also very high. Man has about two million sudorific glands. Let us take another look at sting cells, to see whether the analogy with the kidney and the sweat glands becomes any closer. Perhaps at the same time we shall come to understand why there are no sting cells in sponges. We know t h a t the tegument of polyps is distinguished by the presence of innumerable sting cells, sometimes grouped into batteries. We also know that they can be interiorized, into the stomach for instance, where they live on the food that is swallowed. ~ Organs of attack, of defence, and of nutrition, the sting cells are sensitive and glandular. The bell of a medusa (Gemellaria Implexa) shows on its external surface four pear-shaped pockets full of these sting cells. Here, their presence is difficult to explain; they probably play a part analogous to those poisonous rods which have been demonstrated in certain worms (Tubellaria) and which biologists call "rhabdites". Today these rods are recognized as having the same nature as sting cells. Sting cells can be found interiorized in Solenaucolons (polyps), and Edmond Perrier says t h a t they act to defend the animal against internal parasites. But it is above all in the siphonophores, particularly Disconanthes, that the interiorization of numerous sting cells within the tissues of the animal deserves special notice. *Translated by Dr. M. ttarling, and published by the kind permission of the New Atlantis Foundation. 279
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Between the vault of the gastric cavity and the base of the float of these siphonophores, canals can be seen amongst a colony of sting cells. When the canals leave the gastric cavity for the walls of the float, they cross the peripheral parts of the sting cell colony, and send branches into it which ramify into a network. This network "is considered to be a tubular gland connected with the secretion of digestive juice, and the combined structure has been called the
liver".14 These cells do indeed contain brown granulations which give the whole mass a dark colour, visible through the teguments. The lower canals are different, and their cells contain, not brown granules, but guanine crystals. "This part is thought to be a kidney, but one without special excretory channels." Granted t h a t the mode of transformation of these interiorizing sting cells is unknown, their connections with gastric function and these precursors of the liver and kidney are truly interesting. A certain number of siphonophores eject a coloured excretory fluid which tints the water around them. This secretion is produced b y certain cells of the shield which are basically transformed sting cells, loaded with pigment. Already, amongst the Protozoa, Cannopylaria demonstrate a curious glandular organ, the "Ph~eodium". I t is a pigmented mass whose function is unfortunately still unknown, but in which large cells can be observed which are "distinguishable b y a spirally crossed striation which recalls t h a t of certain nematocysts (sting cells) before they burst". So, far from disappearing, the theme expands . . . The phmodium bears a striking resemblance to the female breast. I n this connection, it should be recalled t h a t the milk-producing gland is no more t h a n a specialized sweat gland. If, behind the complexity of organs (it always comes back to this) we look for function, then i t is evident t h a t the same fundamental theme is being played, here exteriorized, there interiorized, with variations in the different organs which can be shown to have a common source. Thus a study of the sting cell not only shows the connection between the vascular renal glomerulus and the glomerulus of the sudorific gland, and even of the m a m m a r y gland, but also connects the functions of these structures with the function of the liver. B y now it will come quite naturally to us to see t h a t bile is also a product of elimination, even more toxic t h a n urine, to establish t h a t it plays an import a n t role in digestion, to know t h a t it has antiseptic properties, t h a t it disinfects the intestine, and kills the germs which would otherwise invade the biliary passages. Has not Ed. Perrier attributed this latter function to the intcriorized sting cells which defend the animal organism against internal parasites? The transition of an interiorizcd sting cell shows us why it is not found in glands, which are spongy organs, any more t h a n in the sponges themselves. The reason is t h a t '%he sting cell is at once a glandular element and a transformed vibratile cell whose product of secretion is a mucoid and venomous substance".15 I n the transformation caused b y interiorization the vibratilc defence element disappears, whilst the glandular, secretory element develops. I f we examine a drop of liquid from the cavity of the sponge, a cell having amoeboid movement is seen. I t is our old friend the Amoeba, the white cell, if you like, which will appear again in the internal fluid of the echinoderms, and m the blood of the shrimp, the toad, and man. " I f into the space beneath the skin of the back, t h a t is, into the dorsal lymphatic sac of the toad, is injected fluid containing powdered carmine in suspension, and then a fragment of elder-pith is introduced into the same lymphatic sac, it will be found some time later t h a t the cellular cavities of the
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elder-pith are filled with lymphatic cells and that the latter are full of carmine granules. These cells, being very active amcebocytes, have ingested the carmine suspended in the lymph (phagocytosis), and then penetrated into the elder-pith (migration).'16 In higher animals, only the cells derived from embryonic jelly have the property of phagocytosis. In polyps, the cells of the endoderm have it, but amongst the sponges all cells, those of the tegument as well as those of the internal lining, possess this property. We always find it in the cells of the vessel walls in vertebrates, in those lining the renal tubules of annelids, ete . . . . This property is important, for it gives a special character to those cells which possess it. Thus white cells consume all residues, dust, etc. In leeches, the white cells accumulate beneath the skin around foreign bodies, forming a crusty layer. They join battle with microbes, and their bodies form the pus. The earthworms contain white cells which, suspended in the general fluid, are regular little mobile glands filled with globules of unknown nature. The white cells of echinoderms and of bryozoans are floating excretory cells, in the same way as those of certain worms constitute true unicellular glands. In the walls of the intestine the white cells dig themselves holes in which to live, and which are the "blind pits" which function as glands of the intestine. "So", says Prenant, "glandular cells can be seen, now fixed, forming part of solid tissue, now mobile, swimming in their fluid or moving freely throughout the organism; the latter case is that of the red cells of the blood and of the phagocytes." On the other hand, apart from the internal lining and from the capsule-the rind of the sponge--apart from the skeleton, in fact, the substance of the sponge itself is analogous to the interstitial substance of connective tissue. This amorphous substance shows every degree of transparency and of consistency, from nearly fluid jelly to cartilage. Cradled in this jelly, as we have just said, are migrant undifferentiated cells which are identical with amcebse, and with white cells. Now, these migrant cells of the sponge, these interiorized amceb~e, potentially contain the whole sponge, since all parts of the sponge derive from their differentiation, the gland cells of the tegument, and those which produce the skeleton and the sexual elements. These amcebm of the sponge sometimes also assemble in large numbers in certain parts of the body, notably in damaged parts. I f its embryonic origin was of absolute value in demonstrating the significance of an organ or of organisms, this fact alone would reveal the glandular theme of the sponge, according to the principles established in the preceding chapter. But the embryogenic approach is too wide, too large, and leads too quickly to unification. The white cell must be regarded from another point of view to see whether the glandular theme of the sponge can be confirmed. White cells are formed in the lymphatic glands. There, whilst still free lymphatic cells, they can scarcely be distinguished from the fixed cells. Moreover, these lymphatic glands themselves are clusters of elementary nodules formed in the first place by the accumulation of white cells. This formation is complicated by the budding of a lymphatic vessel. " I f this process takes place at the same point in a group of lymphatic vessels running side by side in this or that region, there will be seen to form simultaneously several lymphatic nodes which, through peripheral fusion, constitute the complex lymphatic gland of man. ''17 Thus, the white cell leads us to the lymphatic node, and we see that the complex gland is formed from elements, as is the composite sponge. Parallelism goes farther; the regular lymphatic channels become, in the
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glands of birds, more and more complicated. They become irregular, tortuous, and cavernous in the lymphatic glands of mammals. Finally, the tissue surrounding the gland becomes fibrous, constituting a rind, the capsule. And this is not restricted to the lymphatic glands. "Certain lymphoid organs are formed in the middle of embryonic connective tissue, by concentration of white cells at certain points, and through transformation of the connective tissue into a network. Thus can be roughly represented the formation of a lymphatic gland." Of what use, it could be asked, are the complicated cavities of the gland, any more than those of the sponge. The reasons are identical. In both cases this complication exists to delay the flow of liquid, and to increase the active surface through the production of crypts. I n the lymphatic gland processes can be traced, issuing from the capsule and forming a series of links, which exactly correspond to the sub-cortical sinus of the sponge. The central substance of the lymphatic gland is made of the same tissue as the sinuses; only the links are finer and narrower, and crowded with white cells. I t is at this stage that the gland produces certain of the cells of the blood. I n a sponge all the solid bodies contained in the substance--spicules, algae, etc., are wrapped in a thick coat of connective tissue cells. These cells are very numerous around the reproductive elements, and are, moreover, more tightly packed the nearer they are to the object which they are enveloping. The reproductive elements are constantly developing to the detriment of the amoeboid cells. Those parts which, in the sponge, give rise to the elements of reproduction are differentiated, in the organism, to supply the living elements of the bloodstream.
To recapitulate, allowing for variations in detail, the fundamental structure of the sponge is the same as that of the lymphatic gland. On the other hand, the gland is broached, on its convex surface, by numerous lymphatic vessels, "afferent vessels", which go through the capsule and open into the cortical sinus, and whose walls are continuous with the spongy tissue. Here we are reminded of the system of inhalatory canals of the sponge, which in the sponge of the outside world stop short at the mouth of the inhalatory pore. From the concave surface of the sponge run one or a small number of trunks, the efferent vessels, which correspond to the exhalatory system of the sponges; only, in the spollge, these canals terminate in the same way at the cloacal orifice, whilst in the gland they are prolonged into the lymphatic vessels. I n short, lymph arises at the glands by means of afferent vessels supplied with valves, and enters through the convex surface; it is contained in the large limks of the sub-cortical sinus, and then in the small canicular spaces which separate the more solid masses; it flows around these latter and drains away in all directions through the cavernous channels into the terminal sinuses which we can call "cloacal sinuses". Finally it reaches the excretory orifice on the concave surface and leaves through efferent vessels which are larger and fewer in number than the afferent ones. The basic laws of the sponge can be seen here. Parallelism with the structure of the sponge is, it seems, easy to trace. Just as those nodes also called lymphatic glands are really no more than swellings on the course of lymphatic trunks, this study now leads us quite naturally to that of the lymphatic vascular system as a whole. In a moment we shall have to consider the spleen, which is nothing more than an enormous lymphatic node, and we shall be in a better position to see whether the spongy themes we have uncovered still persist.
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In embryos, lymphatic vessels, as buds from the veins, appear before the lymph nodes. "Thus is verified Ranvier's proposition, t h a t the lymphatic system develops like a gland, an offshoot of the tree of the venous circulation. ''is The lumen of lymphatic vessels is generally fairly wide. When they meet the confluence is much dilated; they even form veritable lacunae in the frog, for instance; and, in connective tissue, canals are formed by lymphatic spaces. Here again we have a renal apparatus, not merely passive, but secretory. The lymphatic canals alone constitute the lymphatic system in fishes and batrachians, which have no lymph nodes. What then is the significance of canals in this case? Professor Prenant will tell us: "The lymphatic system of batrachians, which has become well known through the work of Rasconi and of Panizza, and later through that of Ranvier, is developed in such a way that it can be said of the frog that it is a lymphatic s p o n g e . . . " Here it might be thought that the word sponge is being used in an allusive sense; but this would be an error, for the parallelism is fully justified. In fact, "there exist in these animals capillary lymphatic vessels, fixed, or dilated into lakes in the skin and in different organs, where they form networks (Langer, Ranvier). These original networks are drained by trunk channels, often very short, which open into vast, incompletely partitioned sacs, 'sinuses' or 'lymphatic reservoirs', which are results of the fusion of ordinary capillaries which originally were distinct from one another (Ranvier)." "Thus the capillary networks of the skin communicate by means of very short capillaries with the large sacs, 'subcutaneous sacs', which lie immediately below the tegument. The intestinal lymphatic networks open in the same way into a vast lymphatic reservoir called the 'retroperitoneal cistern', situated under the peritoneum in front of the vertebral column; communication is established by means of capillaries which join one another around the mesenteric arteries, forming 'perivascular lymphatic sheaths'. These great lymphatic sacs had previously been taken for serous spaces not bounded by their own walls. Impregnation with silver has revealed otherwise, that they are bounded by lymphatic endothelium, and a study of their development allows us to hold that they are derived from fused lymphatic vessels. ''19 I t could be said that whereas in the sponge its canals end sharply at their orifices, the fact that lymphatic canals are prolonged into vessels establishes a radical difference. But, as Prenant tells us, if from the morphological point of view the lymphatic system is contained in closed vessels with visible cellular walls, then from the physiological point of view the system of intercellular lacunm of the tissues must be added to this system of canals in order to conceive of the lymphatic system as a whole. The one is a system of preformed channelled routes, the lymphatic routes proper; the other is but a mass of lymph channels lacking both boundaries and permanent existence. Together they represent the complete structure of the sponge. Claude Bernard has said that the anatomical elements of our tissues live in lymph as fish live in water. Indeed, if the blood vascular system is a mechanism of irrigation, that lymphatic system is one of drainage. Its function is to remove from our tissues substances deriving from the blood which has bathed and nourished these same tissues. All this now leads us to consider the serous cavities. The serous cavity is part of the general body cavity in the form of a closed bag (contained between the two layers, parietal and visceral, of the mesoderm). We know that there are three compartments, two thoracic and one abdominal. An extension of the latter is called the vaginal cavity, and contains the testicles.
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The serous membranes, which can be called "lymphatic layers", are all more or less alike, and to understand them the whole peritoneum can be taken as an example. Since their disposition is a m a t t e r of adaptation, it is known t o d a y t h a t their morphology is due to the movement of the organs they surround, rubbing, sliding, e t c . . . At first sight there is nothing about their characteristic formation to suggest the sponges. But none the less, ff only a little trouble be taken, it is not difficult to find, in the midst of detailed modifications, the basic original type. Amongst certain primitive animal species the peritoneum is often ciliated; these cilia m a y also appear in higher forms, in the batrachians for instance, where they guide the egg which has fallen into the peritoneal cavity. As the serous membrane is a sponge, it can contain muscles. I n fact, we do find sub.peritoneal smooth muscle of the broad ligaments, the round ligaments, the superficial part of the uterine musculature... Let us consider the peritoneum in one of its important folds, the great epiploon. I f this fold was originally spongy, how can we trace the complicated system of the canals of the sponge in this membrane ? Will it perhaps be difficult ? B u t nature makes canals very easily. At first the epiploon is a thin membrane formed of two layers of cells; then, there develops between the two a median layer of connective tissue. Later, in the second year (in rabbits), holes appear in the epiploon which, blind at first, later become perforated. In this thin membrane, the holes represent the canals of the sponge. I n m a n the definitive appearance of the epiploon is that of fine lace often with wide interstices due to the passage of white cells. This membrane is extremely mobile; its active movements are not of muscular origin, but are perhaps due to the very large n u m b e r of white cells which it contains. The great epiploon carries to an extreme the scavenging role of its amcebse; thus the abdominal cavity is kept clean. There are in it lymphatic nodules (milky patches) and blood-forming elements which cause it to rate as a veritable gland. " F r o m this it can be concluded t h a t the great cpiploon is, like the spleen, a h~ematopoietic organ; this function could be explained b y the common origin of this membrane and of the spleen (Pardi). ''2o The spleen itself is composed of a fibrous envelope or capsule which sends processes or trabeculse into the interior of the organ. Thus, the spleen is divided into segments. From these trabeculse there arise at certain points other secondary trabeculm which subdivide the segments or lobules into numerous element a r y lobules, in such a way t h a t the organ can be regarded as composed of 500 million elementary lobules. I n the spleen, as in all other organs, blood arrives through the artery and leaves through the vein, thus clearly constituting an afferent or inhalatory system coupled with an efferent or exhalatory one. The walls of the artery gradually fuse with the splenic pulp itself. I n this pulp, which consists chiefly of an infiltration of white cells, bodies are found which formed from thickened tissue filled with the same white cells. The canals lead into areolae which in turn are continued into the enormous exhalatory system of venous canals, where highly contractile "rod" cells are found. I t is through their contraction t h a t blood is driven from the venous sinuses into the veins, a mechanism identical with t h a t which causes the circulation of water in the sponge. The immense venous network of these canals forms a major p a r t of the splenic pulp. From the confluence of the venous sinuses arise little veins which join to form larger trunks which in their turn finally unite to produce the efferent vein. W h a t must be clearly understood is t h a t the arteries and veins of the spleen
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simply represent the bloodstream's routes of arrival and departure in an autonomous organ, whose structure is really that of a dense sponge, and whose active surface is therefore enornous. The more complex functions of the spleen are unfortunately not completely known. This organ is essentially a centre for the destruction of old red cells (debris), and for the production of white cells (amoebae); it has a secretory function, and accumulates carbon, microbes and pigments; it is also a storage place for iron (fixation of colours). L y m p h nodes develop slowly after birth, as the growth of sponges in general is slow. Speaking of lymph nodes, it should be noted that if these tighly-packed masses sometimes appear to have a uniform consistency, more often they show a clear centre, called the "germinative centre", which has been held to be the source of all the different sorts of cells. Now, there is a close analogy between, on the one hand, the formation of white cells and genital cells in the organism, and on the other, between the reproductive and amoeboid elements of the adult sponge. The reproductive elements are generally formed in the genital glands. Now, in polychete worms and nemertids, the jelly covering the peritoneum can produce eggs or spermatozoa from any point on its surface. In the whole class of Gephyrea, the genital organs are but an appendage of the peritoneal membrane, and in some of them the genital elements develop in the peritoneal cavity (general body cavity). Now one can understand why, the peritoneum being fundamentally a sponge, the sexual elements derive from it; the differentiation of sexual glands is but secondary. A fusion can already be seen in the oligochete worms, and in Rana temporaria it is still more obvious that genital cells are produced by progressive differentiation of peritoneal cells. Epithelial cells of the peritoneal membrane can even form placental elements, a fact which has greatly surprised certain authors, though it is really very natural, since the placenta is a spongy organ. The spermatozoa of the sponge appear to develop constantly at the expense of migratory amoeboid ceils. In most of them these amoebae subsequently divide into spermatids which remain grouped; the same amoebze also give rise to female elements which produce eggs. They also remain together. Fertilization of the eggs always takes place inside the sponge, and the e m b r y o develops, either in the canals, or even in the substance of the animal itself. Altogether, it follows from this that white cells in higher animals are no longer able to differentiate into reproductive elements, this function having been taken over by the sexual elements where, as we have seen, atavistic notes sometimes appear, recalling th~ primitive spongy theme. In other words, leaving out the questions of adaptation and homogeneity, the only difference which exists between the sponge and the spleen, for example, is that amoeba~ in the spleen do not transform into reproductive elements. This is a simple question of division of labour which, moreover, has come into operation gradually, as Rana temporaria shows us. Perhaps one could say that blood establishes a major difference between plant.animals and our organs. But one only has to follow its phases in the organism to be aware of the clumsiness of such a concept. Blood leads to lymph, lymph to simple serous fluid, and that by a change in the position of the organ. Serous fluid is like sea-water, and yet in the ox, the peritoneal serous fluid contains more white cells than the blood. Finally, it must be noted that regeneration is as difficult in sponges as it is in glandular organs in general. It is very important to maintain the parallelism which relates the gland,
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the sponge interiorized within an organism, however differentiated and transformed, to the spongy species in nature. We have seen t h a t the internal cavity of sponges is gastric, secretory, excretory, and circulatory. I t is also reproductive, and we could add respiratory, although there is as yet no localized or differentiated respiratory apparatus in sponges. All the functions, in fact, are found there, and it would be erroneous to exclude a n y of them. We cannot close this chapter without taking a look at the skeletal structure of the sponges. There are sponges with calcareous skeletons, sponges with siliceous or horny skeletons, and soft sponges without any skeleton at all. The skeleton is formed b y skeletal needles or spicules. I t is difficult to draw a line of demarcation between the spongy species which we have named. The calcareous spicules are joined b y horny substance,.and correspond up to a point to the collagen fibres and elastic fibres of vertebrates, of which, indeed, they are a simplified form. I t is known, in fact, t h a t structural modifications characterize gelatinous, collagen, and elastic tissues. Spicules can be single, composite, or fused; there are also spicules in striated lamell~e. Others have the form of scales, are mobile, can transform into defensive spines, or on the other hand can be superimposed like the tiles of a roof to act as a protective covering. Certain protozoa already possess analogous structures, but here, according to the species, they become of great importance~ combining wide variation with remarkable stability. Spicules are formed in large granular cells, the "spongioblasts", which in the course of their development turn more or less completely into spicules. In horny sponges the skeleton is made of a substance called "spongin" which bears a certain analogy with silk. Certain tropical sponges have been found to contain iodine in organic combination, giving the substance "iodospongin". To sum up, the spongin which forms the framework of the trabeculse of the sponge is achieved by total or partial transformation of the special cells called "spongioblasts". Now, the framework of the spleen is also formed by the transformation of large granular ceils into a special substance which performs exactly t h e same function as spongin, and which has been called "reticulin". The process of this transformation has been observed in selaeaous fish; and it seems to be almost identical in sponges and in the spleen. Further, in the course of formation of the sponge's skeleton, there appears to be a destruction of spongin performed by special ceils, the "spongioclasts", analogous to "osteoclasts", according to Delage. We shall return to this later. I n a Malayan sponge (Semperella) the skeleton is formed b y a layer of silica uniting the spicules into a rigid grille. This phenomenon, seen in most siliceous sponges, has caused Deluge to say: "This growth could not be better compared than with t h a t of the diaphysis of a long bone, in thickness to the periosteum, and in length to the epiphyseal cartilage, without intercalary growth. ''21 I n Jenthella, the network formed b y the fibres presents s o characteristic a structure t h a t Deluge cannot resist writing, in spite of his horror at the analogy: "The structure of the external layers recalls t h a t of cartilage, except for the regularly concentric arrangement of the cellular elements. ''s2 From another angle, in membranous bone such as the cranial bones, for instance, ossification starts b y the formation of veritable osseous needles, a stage which we can call: spiculous stage. One end of the needle joins others to form the central plaques, whilst the other extremity accounts for the serrate edges of the cranial bones.
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In man, though bony tissue m a y have been laid down during a formative period, it is still a long way from completion. I t s shape and internal structure are due to two contrary processes. I t is, says Duval, as if two architects were working, the one undoing all t h a t the other had done. I n the midst of this apparent chaos, this internal strife, the definitive shape of the bone gradually appears, thanks to a "sculptural resorption". Will this new phenomenon be an exception to the parallelism between spongy and bony structures? The young, fibrous, thick bone is always dense, that is to say, it has no medullary canal. The marrow, a foreign substance with a structure all its own, is destined to find its place within the medullary canal. So who is the architect who will dig this canal? Let us study the sponges to find the answer. Oysters sometimes carry special marks, a sign of what has been called "spicebread disease". This peculiarity, which does not seem to be very harmful to oysters, is due to the presence of a curious sponge, Cliona. This sponge usually lives inside calcareous objects, such as pebbles and the shells of molluscs; externally it only shows as small points corresponding with the orifices which communicate with the outside world. This is what Delage says: "The interior part forms a network, contained in the galleries which the sponge hollows out in its host. This is the reticulate form. I n certain cases, where the host has become riddled throughout its length, the sponge, in order to go on growing, overlaps the end and turns back on itself. I n the end it m a y undergo such considerable development t h a t the host, engulfed, disappears; it then takes a massive form. ''23 I n the light of interiorization does not this description suggest the process of bone formation? especially if the last stages are considered as exaggerations which are found in the organism under pathological conditions? But this is not all. Just in the same way t h a t bone marrow, though it is spongy tissue, is not typical of glands in general, so this sponge seems to be an exception to the basic law. The relatively very large canals are only indifferently exhalatory or inhalatory. Moreover, Clionas, of which there are m a n y varieties, are not the only perforating sponges. There are others which have a siliceous skeleton, whilst Oscarella, for instance, has no skeleton at all. T o sum up, perforating sponges develop the osteoclastic theme which has already been noticed, and represent the destructive architect who is indispensable to bone formation. These thoughts on bone force us to recall the polyps, amongst which we have already found spicules and osseous notes. Bone, therefore, represents a combination of polyp and spongy themes. Now, in this combination, the perforating sponge at least, if not all of the sponges, stands, as we have seen, for that other architect of whom Professor D u r a l speaks, he who is in constant conflict with the first, constructive architect. Will this first architect be found in the camp of the polyps? I f we are right, we shall find opposition between sponges and polyps. I n fact, we are saying t h a t "contrary to the universal rule for sponges, the ciliated ectoderm is invaginated into the endoderm (in the transformation of the gastrula), so t h a t in the adult the normal relationships of the layers are reversed ~' .24 I n other words, the sponge, from certain points of view, is like a glove turned inside out, a polyp back to front, whose epidermis has become its internal lining, and whose collar cells derive from the transformation of tegumentary cells. Whether this reversal of invagination in the larvm of the ancestors of sponges
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was due to mechanical causes or not does not m a t t e r much. I t is only necessary to remember the existence of this opposition which manifests both in the absence of regeneration, and in the completely internal activity of the sponge. Although t h e y have no eyes, the larvm of siliceous sponges, sensitive to light, seek darkness, whilst the green hydra seeks light for its algae, just as the shoot of a tree seeks it for itself. Polyps extending their agile tentacles, medusm scudding on the waves, both show an external activity. This opposition which has been pointed out b y Y. Delage, is not absolute; on the contrary, more often the two forms complement one another. Let us look at them again, in the organism and in nature. The blood vascular apparatus merges imperceptibly into the lymphatic system. I n batrachians and reptiles, as in certain fishes and birds, there are, in the course of the lymphatic current, organs endowed, as is the heart, with rhythmical contractions; they are called "lymphatic hearts". Thus, here also we find fused notes. I n the course of vessels are found spongy organs; spleen, lymph nodes; so t h a t it is natural to find inside the bones themselves marrow, a spongy substance. These complicated fused notes cannot be attributed exclusively to polyps or to sponges; really they belong to both. Classification exists principally in our minds.
Nonetheless, it must be recognized that polypian characteristics predominate in vesse/s, spongy characteristics in glands. This is what we wanted to establish. At the same time, this is how the internal reality of the organism can be approached, the reality of an incessant activity which is killed by immobilizing words. I t is, in fact, an association, a joining, a complication, a perpetual process of mutual help as well as a constant struggle. Red cells originate in the organs which will destroy them, bone is not only the result of the battles which we have indicated, it owes its shape to opposing pulls. Cartilage has already had to disappear to make way for it, and the point of ossification, primitively the point of calcification, sees its walls founder where the vessel penetrates. Polyps and sponges thus seem to be interiorized within the organism like fundamental animals, that is to say, representing the primitive materials of all our organs. Their themes become more marked in pathological phenomena. Think only of the gelatinous plaques which, developing on arteries, become infiltrated with calcium, of the calculi of all sorts which appear in the salivary glands and in the kidneys, and it will be seen t h a t if the deposition of calcium is normally restricted to special areas, primitive tendencies and atavistic memories can awaken out of time, and by sharp pain remind us of their original character. Up till now we have considered polyps and sponges as opposed to one another. But this is only relative, and there can also be strong sympathies between these animal groups. The deep sea fauna includes creatures upon whose classification authors are not agreed. Those which are held b y some to be sponges, are for others gigantic colonies of protozoans, or again polyps living in symbiosis with sponges. All authors, and this is very important, have noted the similarity between sponges and coral polyps, osteoid polyps we should say, since this group shows the chief osseous note. Hyalonema is a sponge shaped like the calyx of a flower, borne on a stalk 30 centimetres high. On this stalk a polyp becomes fixed, so constantly t h a t it was thought to be the skeleton. More and more new cases of symbiosis are found . . . The opposition between polyp and sponge is thus quite relative. Almost always we find them side b y side in nature, often exchanging functions. We
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can go further. Polyp and sponge have a tendency to unite, even whilst o p p o s i t i o n e x i s t s b e t w e e n t h e m , so t h a t w e c a n b e s t e x p r e s s t h e i r r e c i p r o c a l r e l a t i o n s h i p s as r e p e a t i n g , : i n a n o t h e r m e d i u m , t h e t w o g r e a t m a n i f e s t a t i o n s of the vegetable world: polyp, the stalk; sponge, the root. REFERENCES
12 Delage, Yves, et Herouard, Edgard, Traitd de Zoologie concrete, Vol. VIII, p. 313. Sehleicher freres, Paris, 1899. la Jaworski, H., Pourquoi la Mort? pp. 48, 73, 224. 14 Delage, Yves, Op. cir., Vol. II, p. 256. 1~ Prenant, A., Bouin, P., et Maillard, L., Traitd d'Histologie, Vol. I, p. 178. Masson et Cie, Paris, 1911. le Prenant, A., Op. cir., Vol. I, pp. 543, 544. 1~ Branca, J., Prdcis d'Histologie, p. 327. Bailli~re et ills, Paris, 1914. is Prenant, A., Op. cit., Vol. II, p. 118. 1D Prenant, A., Op. cir., Vol. II, p. 122. 20 Prenant, A., Op. cir., Vol. II, p. 305. ~1 Delage, Yves, Op. cit., Vol. II, p. 131. ~2 Delage, Yves, Op. cit., Vol. II, p. 142. 2a Delage, Yves, Op. cit., Vol. II, p. 173. 2a Delage, Yves, Op. cir., Vol. II, p. 61.