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Prey acquisition in atectal frogs
Brain Research, 153 (1978) 217-221 © Elsevier/North-Holland Biomedical Pres~
217
Prey acquisition in atectal frogs
CHRISTOPHER COMER and PAUL GROBSTEIN
Department of Biology, The University o[ Chicago, Chicago, Ill. 60637 and Department of Pharmacological and Physiological Sciences, The University of Chicago, Chicago, Ill. 60637 (U.S.A.) (Accepted April 6th, 1978)
The midbrain tectum of vertebrates has long been implicated in the mediation of visual orienting behavior, perhaps the classic example being the prey acquisition responses of anuran amphibians4, 6. Tectal ablations have consistently been reported to abolish this behavior1, az. Recent neurophysiological studies have demonstrated that in addition to visual information, tactile and auditory information reach the optic tectum in a wide variety of vertebrates 2,3,8,14-16, including the frogS, v. We have recently observed that blinded frogs exhibit prey acquisition responses following appropriate tactile stimulation, and have therefore tested the possibility that the optic tectum is essential for orienting responses regardless of the sensory modality eliciting the behavior. We report here that oriented prey strikes elicited by tactile stimulation persist following removal of the entire optic tectum. Commercially obtained adult leopard frogs (Rana pipiens) were used in all experiments. Surgery was performed under tricaine methanesulfonate anesthesia. The experimental group consisted of frogs preselected for reliable responsiveness to mealworms presented in a standardized visual perimetry test. Following tectal aspiration lesions, animals were retested by perimetry to assay the extent of visual deficits. Particular attention was paid to the lower visual field. Extensive tests were then made of tactile responses at 5 different body positions (see Table I). Testing was completed within 3 weeks following the lesion. At the end of this period, the optic nerves were sectioned bilaterally and testing of tactile behavior was repeated over the two weeks following section. At the conclusion of the experiments, the extent of the tectal lesions was assayed histologically from 20 #m coronal sections stained with cresyl violet. For this purpose, we define 'tectum' as the tissue of the dorsal midbrain displaying a typical laminated cytoarchitecture. Much of this tissue is unambiguously identified by its location above the tectal ventricle. Successful lesions were carried far enough ventrolaterally and anteroventrally to eliminate the typical lamination. The tactile behavior observed in blind but otherwise normal frogs will be reported in detail elsewhere (Comer, in preparation) but our method for assaying it can be briefly described here. Light stroking with a pipe cleaner under the chin provokes
218 TABLE 1
Comparison o f the tactile orienting behavior o f a blind but otherwise intact Jrog with that ot all atectal frog before and after optic nerve section
The 5 stimulus positions listed at left were tested in a r a n d o m sequence by gentle stroking with the edge o f a pipe cleaner. The frog was in the center o f a 360 ° p r o t r a c t o r and the angular deviation of the s nout d u r i n g s n a p p i n g or at the completion of a turn was recorded as the turn amplitude. Snaps elicited followin g stimulation of the snout (under the chin) are directed forward and therefore no t urn a m p l i t u d e is recorded. A trial consisted of a m a x i m u m o f 5 strokes. If the s ki n was stroked 5 times with no response, then this failure to respond was recorded. F o r the n o r m a l frog there were 16 trials at each position distributed over 8 days. F o r the atectal frog the formal testing consisted o f 30 trials at each position distributed over 10 days b e g i n n i n g 9 days after the lesion. Responses had, however, already been observed at the first i n f o r m a l testing 3 days after the lesion. Formal tactile testing was repeated over a 10 day p e r i o d b e g i n n i n g two days after optic nerve section. The atectal animal whose behavior is s h o w n in this table is the same frog whose lesion is illustrated in Fig. I. Normal Optic' nerve sectioned
Snout Left forefoot Right forefoot Left h i n d f o o t Right h i n d f o o t
Atectal Before optic nerve section
Atectal After optic her ve section
Percent responses
Turn amplitude Percent mean 5-S.E.M. responses
Turn amplitude Percent mean 5-S.E.M. responses
Turn amplitade mean 5-S.E.M.
100 100 100 100 100
--20.0 ~' 3~ 18.7 ~' ~ 1 2 0 . 3 5122.8 ° ~
--1 5 . 0 _L 5.ff 48.6 ~ 5- 2.4 ° 49.2 ± 3.7
13,5 I.I 18.5 + 1 . 0 44,3' 3.0 49.2 t 3 . 9
1.4' 1.3 ~ 5.7' 5.0
0 0 6.7 99.3 80.0
6.7 33.3 66.7 70.0 63.3
a forward snap. Similar stimuli delivered to the forelimb elicit a snap directed at the forelimb. By contrast, stimuli delivered to the flank, hindtimb or vent do not elicit a snap but instead cause the animal to turn by an amount appropriately related tothe site of stimulation. This tactually elicited behavior of a frog is thus spatially organized much as is the prey catching response of a normal frog to appropriate visual stimuli. These responses are sufficient to allow a blinded frog to successfully capture live prey items which touch its skin and are often present within one day of optic nerve section. Of 6 animals in which complete bilateral lesions were attempted, only two subsequently showed a complete absence of visual responses to prey items in all parts of the visual field. Nonetheless, both frogs did produce oriented snaps when rostral body positions were stimulated and turns of the body when caudal sites were stimulated. In both frogs responses were seen at first testing 3 days after the lesion. Histological examination of these frogs' brains revealed a complete absence of any tectal tissue with slight damage to the underlying torus semicircularis, as illustrated for one animal in Fig. 1. Similar damage was sustained by the other animal. Thus oriented prey strikes can be elicited from frogs which are anatomically atectal and which also display the complete absence of visually elicited prey acquisition behavior which functionally characterizes the atectal animal. We have observed that frogs with tectal lesions usually respond less frequently to tactile stimulation than control animals. In addition, some lesioned frogs tended to produce head and body turns that would 'undershoot' the normal responses seen for
219
Fig. 1. a: reconstruction of the lesion for the atectal frog whose behavior is illustrated in Table 1. The extent of tissue removal is indicated by shading on a series of five schematic sections of the midbrain which are based on normal material. Rostral is toward the reader. Tectal lamination is indicated by a dotted line corresponding to stratum griseum centrale and a closed band corresponding to stratum griseum periventriculare, pt, pretectal nucleus; ts, torus semicircularis; ni, nucleus isthmi, b: photomicrograph of one of the sections from which the reconstruction was made. The section is at a level which approximately corresponds to the second most caudal section of the reconstruction.
220 a given body position. This can easily be seen in Table I where the mean turn amplitudes and response frequencies for the atectal frog whose lesion is illustrated in Fig. l are compared with those for a blind but otherwise normal frog. The reason for this undershooting and lower response frequency is not yet clear. It may be due to whatever subtectal damage our frogs did receive, or alternatively it may indicate that the optic tectum has some modulating influence on tactually elicited prey acquisition responses. As can also be seen in Table I, response frequencies for anterior stimulation tended to be lower than for posterior for the atectal animal. To some extent this may reflect the use of a more rigorous criterion for a response to rostral stimulation (a snap) than for a response to caudal stimulation (a turn). On the other hand, it may indicate a greater involvement of dorsal midbrain in responsiveness to tactile stimulation for rostral as opposed to caudal body surface. The cell bodies of the mesencephalic nucleus of the trigeminal nerve are located in the tectum 10 and so this system was certainly disrupted by our lesions. Responses to rostral stimulation from our atectal animals clearly show however that this system is not essential for tactile sensitivity. The behavior reported here was noted briefly by Kicliter 9 during the course of a lesion study of visual behavior in the frog. His observations, however, seemed to indicate that tactile prey strikes could be abolished by complete tectal lesions. This apparent discrepancy with the results reported here may be explained by the fact that lesions in the present study were nearly completely restricted to the optic tectum whereas those in Kicliter's study appear to have included considerable subtectat tissue. Preliminary evidence from other experiments in our laboratory indicates that subtectal damage may indeed produce severe and lasting deficits in tactile orienting responses (Comer and Grobstein, unpublished data). Another explanation for the apparent discrepancy is suggested by our own data. In some cases tectally lesioned frogs did not respond reliably to tactile stimulation until after the optic nerves were sectioned. Such behavior was exhibited by one of the two atectal frogs and is illustrated by the data presented in Table [. This suggests that non-tectal areas receiving retinal input may have some inhibitory influence on the production of tactile orienting behavior. As Kicliter's frogs had intact optic nerves they might have been subject to such an effect. This intriguing possibility will require further investigation. It is clear from the results reported here that tectal ablation differentially affects prey acquisition behavior elicited via visual as opposed to tactile inputs. We conclude that, in the frog at least, polymodal sensory convergence does not mean that tectum plays the same functional role in producing spatially organized motor behavior regardless of sensory channel activated. It is somewhat difficult to compare our results to those in other organisms because of problems in relating tectal laminae, possible differences in the degree and time course of post-lesion recovery, and variations in behavioral assays. However, a conclusion similar to ours for the frog seems to apply in the case of the hamster where undercutting the tectum renders the animal unable to orient to visual stimuli but relatively unimpaired in turning toward and taking food which touches the whiskers 11. We should stress that, while we have emphasized the substantial difference in the effect of a tectal lesion on visual as opposed to tactile orienting, our observations do document some abnormalities in tactile orienting. Our results, then,
221 are consistent with the conclusion of studies in m a m m a l s that tectal lesions may produce abnormalities in orienting responses to stimuli of several different modalities 11,13. It is also clear from the results reported here that tectal ablation does not abolish the ability of the frog to generate spatially organized prey acquisition behavior. This implies that pre-motor circuitry capable of elaborating such behavior is present outside the tectum. The possibility exists that such circuitry is involved as well in visually elicited prey acquisition behavior, via a projection from tectum. Whether one refers to tectum as a sensory or a 'sensorimotor' structure is a matter of semantics. It is, however, worth stressing that the tectum is not necessary to the production of prey acquisition behavior. The effects of tectal lesions on visual prey acquisition behavior could then be a t t r i b u t a b l e largely to the interruption of the sensory pathway. This research was supported by PHS G r a n t s EY-01658 and EY-00057 and an Alfred P. Sloan Research Fellowship to P. G. We thank M. Lem for technical assistance and C. Bailey for secretarial assistance.
I Bechterew, W., U ber die Function der Vierhugel, Pfliigers Arch. ges. Physiol., 33 (1884) 413--439. 2 Cotter, J. R., Visual and nonvisual units recorded from the optic rectum of Gallus domesticus, Brain Behav. Evol., 13 (1976) 1 21. 3 Dr~iger, U. C. and Hubel, D. H., Responses to visual stimulation and relationship between visual, auditory and somatosensory inputs in mouse superior colliculus, J. Neurophysiol., 38 (1975) 690713. 4 Ewert, J.-P., Neural mechanisms of prey catching and avoidance behavior in the toad (Bufi~ bl([i~), Brain Behav. Evol., 3 (1970) 36 56. 5 Fite, K. V., Single unit analysis of binocular neurons in the frog optic tectum, Exp. Neurol., 24 (1969) 475-486. 6 Ingle, D., Visuomotor functions of the frog optic tectum, Braht Behav. Evol., 3 (1970) 57-71. 7 Ingle, D., The visual system, In B. Lofts (Ed.), Physiology of the Amphibia, Vol. 111, Academic Press, New York, 1976, pp. 421-441. 8 Jassik-Gerschenfeld, D., Somesthetic and visual responses of superior colliculus neurones, Nature (Lond.), 208 (1965) 898-900. 9 Kicliter, E., Flux, wavelength, and movement discrimination in frogs: forebrain and midbrain contributions, Brain Behav. Evol., 8 (1973) 340 365. 10 Rubinson, D., Connections of the mesencephalic nucleus of the trigeminal nerve in the frog, Brain Research, 19 (1970) 3 14. 11 Schneider, G. E., Two visual systems, Science, 153 (1969) 895 902. 12 Sperry, R. W., Optic nerve regeneration with return of vision in anurans, J. Neurophysiol., 7 (1944) 57-69. 13 Sprague, J. M. and Meikle, T. H., The role of the superior colliculus in visually guided behavior, Exp. neurol., 11 (1965) 115 146 . 14 Stein, B. E., Magalhaes-Castro, B. and Kruger, L., Superior colliculus: visuotopic-somatopic overlap, Science, 189 (1975) 224~225. 15 Tiao, Y.-C. and Blakemore, C., Functional organization in the superior colliculus of the golden hamster, J. comp. NeuroL, 168 (1976) 483-504. 16 Wickelgren, B. G., Superior colliculus: some receptive field properties of bimodally responsive cells, Science, 173 (1971) 69 72.
217
Prey acquisition in atectal frogs
CHRISTOPHER COMER and PAUL GROBSTEIN
Department of Biology, The University o[ Chicago, Chicago, Ill. 60637 and Department of Pharmacological and Physiological Sciences, The University of Chicago, Chicago, Ill. 60637 (U.S.A.) (Accepted April 6th, 1978)
The midbrain tectum of vertebrates has long been implicated in the mediation of visual orienting behavior, perhaps the classic example being the prey acquisition responses of anuran amphibians4, 6. Tectal ablations have consistently been reported to abolish this behavior1, az. Recent neurophysiological studies have demonstrated that in addition to visual information, tactile and auditory information reach the optic tectum in a wide variety of vertebrates 2,3,8,14-16, including the frogS, v. We have recently observed that blinded frogs exhibit prey acquisition responses following appropriate tactile stimulation, and have therefore tested the possibility that the optic tectum is essential for orienting responses regardless of the sensory modality eliciting the behavior. We report here that oriented prey strikes elicited by tactile stimulation persist following removal of the entire optic tectum. Commercially obtained adult leopard frogs (Rana pipiens) were used in all experiments. Surgery was performed under tricaine methanesulfonate anesthesia. The experimental group consisted of frogs preselected for reliable responsiveness to mealworms presented in a standardized visual perimetry test. Following tectal aspiration lesions, animals were retested by perimetry to assay the extent of visual deficits. Particular attention was paid to the lower visual field. Extensive tests were then made of tactile responses at 5 different body positions (see Table I). Testing was completed within 3 weeks following the lesion. At the end of this period, the optic nerves were sectioned bilaterally and testing of tactile behavior was repeated over the two weeks following section. At the conclusion of the experiments, the extent of the tectal lesions was assayed histologically from 20 #m coronal sections stained with cresyl violet. For this purpose, we define 'tectum' as the tissue of the dorsal midbrain displaying a typical laminated cytoarchitecture. Much of this tissue is unambiguously identified by its location above the tectal ventricle. Successful lesions were carried far enough ventrolaterally and anteroventrally to eliminate the typical lamination. The tactile behavior observed in blind but otherwise normal frogs will be reported in detail elsewhere (Comer, in preparation) but our method for assaying it can be briefly described here. Light stroking with a pipe cleaner under the chin provokes
218 TABLE 1
Comparison o f the tactile orienting behavior o f a blind but otherwise intact Jrog with that ot all atectal frog before and after optic nerve section
The 5 stimulus positions listed at left were tested in a r a n d o m sequence by gentle stroking with the edge o f a pipe cleaner. The frog was in the center o f a 360 ° p r o t r a c t o r and the angular deviation of the s nout d u r i n g s n a p p i n g or at the completion of a turn was recorded as the turn amplitude. Snaps elicited followin g stimulation of the snout (under the chin) are directed forward and therefore no t urn a m p l i t u d e is recorded. A trial consisted of a m a x i m u m o f 5 strokes. If the s ki n was stroked 5 times with no response, then this failure to respond was recorded. F o r the n o r m a l frog there were 16 trials at each position distributed over 8 days. F o r the atectal frog the formal testing consisted o f 30 trials at each position distributed over 10 days b e g i n n i n g 9 days after the lesion. Responses had, however, already been observed at the first i n f o r m a l testing 3 days after the lesion. Formal tactile testing was repeated over a 10 day p e r i o d b e g i n n i n g two days after optic nerve section. The atectal animal whose behavior is s h o w n in this table is the same frog whose lesion is illustrated in Fig. I. Normal Optic' nerve sectioned
Snout Left forefoot Right forefoot Left h i n d f o o t Right h i n d f o o t
Atectal Before optic nerve section
Atectal After optic her ve section
Percent responses
Turn amplitude Percent mean 5-S.E.M. responses
Turn amplitude Percent mean 5-S.E.M. responses
Turn amplitade mean 5-S.E.M.
100 100 100 100 100
--20.0 ~' 3~ 18.7 ~' ~ 1 2 0 . 3 5122.8 ° ~
--1 5 . 0 _L 5.ff 48.6 ~ 5- 2.4 ° 49.2 ± 3.7
13,5 I.I 18.5 + 1 . 0 44,3' 3.0 49.2 t 3 . 9
1.4' 1.3 ~ 5.7' 5.0
0 0 6.7 99.3 80.0
6.7 33.3 66.7 70.0 63.3
a forward snap. Similar stimuli delivered to the forelimb elicit a snap directed at the forelimb. By contrast, stimuli delivered to the flank, hindtimb or vent do not elicit a snap but instead cause the animal to turn by an amount appropriately related tothe site of stimulation. This tactually elicited behavior of a frog is thus spatially organized much as is the prey catching response of a normal frog to appropriate visual stimuli. These responses are sufficient to allow a blinded frog to successfully capture live prey items which touch its skin and are often present within one day of optic nerve section. Of 6 animals in which complete bilateral lesions were attempted, only two subsequently showed a complete absence of visual responses to prey items in all parts of the visual field. Nonetheless, both frogs did produce oriented snaps when rostral body positions were stimulated and turns of the body when caudal sites were stimulated. In both frogs responses were seen at first testing 3 days after the lesion. Histological examination of these frogs' brains revealed a complete absence of any tectal tissue with slight damage to the underlying torus semicircularis, as illustrated for one animal in Fig. 1. Similar damage was sustained by the other animal. Thus oriented prey strikes can be elicited from frogs which are anatomically atectal and which also display the complete absence of visually elicited prey acquisition behavior which functionally characterizes the atectal animal. We have observed that frogs with tectal lesions usually respond less frequently to tactile stimulation than control animals. In addition, some lesioned frogs tended to produce head and body turns that would 'undershoot' the normal responses seen for
219
Fig. 1. a: reconstruction of the lesion for the atectal frog whose behavior is illustrated in Table 1. The extent of tissue removal is indicated by shading on a series of five schematic sections of the midbrain which are based on normal material. Rostral is toward the reader. Tectal lamination is indicated by a dotted line corresponding to stratum griseum centrale and a closed band corresponding to stratum griseum periventriculare, pt, pretectal nucleus; ts, torus semicircularis; ni, nucleus isthmi, b: photomicrograph of one of the sections from which the reconstruction was made. The section is at a level which approximately corresponds to the second most caudal section of the reconstruction.
220 a given body position. This can easily be seen in Table I where the mean turn amplitudes and response frequencies for the atectal frog whose lesion is illustrated in Fig. l are compared with those for a blind but otherwise normal frog. The reason for this undershooting and lower response frequency is not yet clear. It may be due to whatever subtectal damage our frogs did receive, or alternatively it may indicate that the optic tectum has some modulating influence on tactually elicited prey acquisition responses. As can also be seen in Table I, response frequencies for anterior stimulation tended to be lower than for posterior for the atectal animal. To some extent this may reflect the use of a more rigorous criterion for a response to rostral stimulation (a snap) than for a response to caudal stimulation (a turn). On the other hand, it may indicate a greater involvement of dorsal midbrain in responsiveness to tactile stimulation for rostral as opposed to caudal body surface. The cell bodies of the mesencephalic nucleus of the trigeminal nerve are located in the tectum 10 and so this system was certainly disrupted by our lesions. Responses to rostral stimulation from our atectal animals clearly show however that this system is not essential for tactile sensitivity. The behavior reported here was noted briefly by Kicliter 9 during the course of a lesion study of visual behavior in the frog. His observations, however, seemed to indicate that tactile prey strikes could be abolished by complete tectal lesions. This apparent discrepancy with the results reported here may be explained by the fact that lesions in the present study were nearly completely restricted to the optic tectum whereas those in Kicliter's study appear to have included considerable subtectat tissue. Preliminary evidence from other experiments in our laboratory indicates that subtectal damage may indeed produce severe and lasting deficits in tactile orienting responses (Comer and Grobstein, unpublished data). Another explanation for the apparent discrepancy is suggested by our own data. In some cases tectally lesioned frogs did not respond reliably to tactile stimulation until after the optic nerves were sectioned. Such behavior was exhibited by one of the two atectal frogs and is illustrated by the data presented in Table [. This suggests that non-tectal areas receiving retinal input may have some inhibitory influence on the production of tactile orienting behavior. As Kicliter's frogs had intact optic nerves they might have been subject to such an effect. This intriguing possibility will require further investigation. It is clear from the results reported here that tectal ablation differentially affects prey acquisition behavior elicited via visual as opposed to tactile inputs. We conclude that, in the frog at least, polymodal sensory convergence does not mean that tectum plays the same functional role in producing spatially organized motor behavior regardless of sensory channel activated. It is somewhat difficult to compare our results to those in other organisms because of problems in relating tectal laminae, possible differences in the degree and time course of post-lesion recovery, and variations in behavioral assays. However, a conclusion similar to ours for the frog seems to apply in the case of the hamster where undercutting the tectum renders the animal unable to orient to visual stimuli but relatively unimpaired in turning toward and taking food which touches the whiskers 11. We should stress that, while we have emphasized the substantial difference in the effect of a tectal lesion on visual as opposed to tactile orienting, our observations do document some abnormalities in tactile orienting. Our results, then,
221 are consistent with the conclusion of studies in m a m m a l s that tectal lesions may produce abnormalities in orienting responses to stimuli of several different modalities 11,13. It is also clear from the results reported here that tectal ablation does not abolish the ability of the frog to generate spatially organized prey acquisition behavior. This implies that pre-motor circuitry capable of elaborating such behavior is present outside the tectum. The possibility exists that such circuitry is involved as well in visually elicited prey acquisition behavior, via a projection from tectum. Whether one refers to tectum as a sensory or a 'sensorimotor' structure is a matter of semantics. It is, however, worth stressing that the tectum is not necessary to the production of prey acquisition behavior. The effects of tectal lesions on visual prey acquisition behavior could then be a t t r i b u t a b l e largely to the interruption of the sensory pathway. This research was supported by PHS G r a n t s EY-01658 and EY-00057 and an Alfred P. Sloan Research Fellowship to P. G. We thank M. Lem for technical assistance and C. Bailey for secretarial assistance.
I Bechterew, W., U ber die Function der Vierhugel, Pfliigers Arch. ges. Physiol., 33 (1884) 413--439. 2 Cotter, J. R., Visual and nonvisual units recorded from the optic rectum of Gallus domesticus, Brain Behav. Evol., 13 (1976) 1 21. 3 Dr~iger, U. C. and Hubel, D. H., Responses to visual stimulation and relationship between visual, auditory and somatosensory inputs in mouse superior colliculus, J. Neurophysiol., 38 (1975) 690713. 4 Ewert, J.-P., Neural mechanisms of prey catching and avoidance behavior in the toad (Bufi~ bl([i~), Brain Behav. Evol., 3 (1970) 36 56. 5 Fite, K. V., Single unit analysis of binocular neurons in the frog optic tectum, Exp. Neurol., 24 (1969) 475-486. 6 Ingle, D., Visuomotor functions of the frog optic tectum, Braht Behav. Evol., 3 (1970) 57-71. 7 Ingle, D., The visual system, In B. Lofts (Ed.), Physiology of the Amphibia, Vol. 111, Academic Press, New York, 1976, pp. 421-441. 8 Jassik-Gerschenfeld, D., Somesthetic and visual responses of superior colliculus neurones, Nature (Lond.), 208 (1965) 898-900. 9 Kicliter, E., Flux, wavelength, and movement discrimination in frogs: forebrain and midbrain contributions, Brain Behav. Evol., 8 (1973) 340 365. 10 Rubinson, D., Connections of the mesencephalic nucleus of the trigeminal nerve in the frog, Brain Research, 19 (1970) 3 14. 11 Schneider, G. E., Two visual systems, Science, 153 (1969) 895 902. 12 Sperry, R. W., Optic nerve regeneration with return of vision in anurans, J. Neurophysiol., 7 (1944) 57-69. 13 Sprague, J. M. and Meikle, T. H., The role of the superior colliculus in visually guided behavior, Exp. neurol., 11 (1965) 115 146 . 14 Stein, B. E., Magalhaes-Castro, B. and Kruger, L., Superior colliculus: visuotopic-somatopic overlap, Science, 189 (1975) 224~225. 15 Tiao, Y.-C. and Blakemore, C., Functional organization in the superior colliculus of the golden hamster, J. comp. NeuroL, 168 (1976) 483-504. 16 Wickelgren, B. G., Superior colliculus: some receptive field properties of bimodally responsive cells, Science, 173 (1971) 69 72.