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The Influence of Egg Weight on the Pre-Hatching and Post-Hatching Growth Rate in the Fowl
T h e Influence of Egg Weight on the Pre-Hatching and Post-Hatching Growth Rate in the Fowl I. EGG WEIGHT-EMBRYONIC DEVELOPMENT RATIOS 12 WILLIAM H. WILEY 8 Rhode Island Agricultural Experiment Station, Kingston, Rhode Island
F
OR years it has been known that egg weight exerts a control on the weight of baby chicks at hatching time. A small egg produces a small chick, large egg produces a large chick. One would be apt to assume that such a start in life extends back into embryonic development prior to the hatching of the egg. Further, one might assume that any disadvantage suffered by the chick hatched from a small egg would be continued on through to maturity. Evidence was presented by Benjamin (1920), Hablersleben and Mussehl (1922), Upp (1928), Graham (1931), and Munroe and Kosin (1940) which showed a highly significant, positive relationship between egg weight and chick weight at hatching time. JuU and Quinn (1925) reported an influence of egg weight on body weight differences between male and female chicks, but contrasting results were published by Munroe and Kosin (1940) who 1 Contribution No. 745 of the Rhode Island Agricultural Experiment Station, Kingston, Rhode Island. 2 A condensation of a portion of a thesis submitted to the graduate faculty at the A. & M. College of Texas in partial fulfillment of the requirements for the degree of Doctor of Philosophy. 3 Formerly at the University of Arkansas, Department of Animal Industry, Fayetteville, Arkansas.
570
used percent of egg weight in expressing body weight of the chicks. One object of this study was to determine if possible, where egg weight exerted its first influence upon the rate of growth of the chick during embryonic development. Another objective was to determine the influence of egg weight on the size and number of cells in a given field of embryonic tissue. Though little work has been reported concerning eggs of different weights, considerable research has been carried out in the study of cell size and numbers as they affect the weight of individuals. According to Painter (1928), genetic factors are expressed early in life through the cell division differential. He began with segmenting rabbit eggs of the same size. There were more cells in larger embryos than in small ones, but there was no difference in nuclear size. Castle (1929) concurred with Painter when he gave the hereditary basis of large and small body size in rabbits as the differential rate of cell multiplication and growth. Castle and Gregory (1929) showed that the more rapid rate of cell multiplication in larger-race rabbits was transmitted equally by sperm and egg. Castle and Gregory (1931) found no significant difference in blastomere numbers between large- and small-race rabbits
Downloaded from http://ps.oxfordjournals.org/ at University of North Dakota on June 24, 2015
(Received for publication February 23, 1950)
INFLUENCE OF EGG WEIGHT ON GROWTH RATE
MATERIALS AND METHODS
This study was divided into two stages, an early setting (fall) and a late setting (spring). The stages of embryological development studied were 72 hours, 14 days, and 19 days of incubation. These periods were adopted for use since Romanoff (1929) considered them growth-constant periods. This work was started in November, 1946 and completed in February, 1948. Barred Plymouth Rock parent stocks were used which had been selected for the production of large eggs (57 grams or more) or for the production of small eggs (52 grams or less). The eggs used were collected each hour, marked and placed in a room cool enough to prevent continued cleavage division of cells. The eggs were weighed to the nearest 0.5 gram just prior
to setting. In an attempt to maintain an even oviduct-retention time, only eggs collected between 8:00 a.m. and 4:30 p.m. were set. During the incubation period, standard temperature and humidity were maintained. Paired samples of eggs were set up in such a way that two eggs, one large and one small, were set side by side at ten-minute intervals. This time interval permitted preparing and fixing each pair of embryos before the next pair was ready to be removed from the machine. Warm Bouin's fluid was used for fixing the embryos. Standard techniques were followed for dehydrating, clearing, and embedding the embryos in paraffin. Each 72-hour embryo was serially cross-sectioned at 7 n on a rotary microtome and placed on slides. The tissues were stained with Harris' haematoxylin and counterstained with eosin. Permanent slides were made of the desired portion of the embryos. Data were collected on the 72-hour embryos as to the number and the size of individual cells in a given area of the optic stalk. Determinations of cell numbers were made with an ocular disc micrometer divided into 0.1 mm. squares. Four adjacent squares were used as a field, and all cells were counted which had as much as one-half their nuclear area inside the outer margin of the squares. Ten fields were counted in each embryo. A magnification of 430X was used in the cell size and number determinations. The cell measurements were made with an adjustable ocular micrometer. In handling the 14-day and 19-day groups the dried embryos were weighed to the nearest 0.5 gram and only the left thigh was used for fixing. Cells were counted and measured in cross sections of the anterior end of the gastrocnemius muscle. All preparation was the same as that in the 72-hour group.
Downloaded from http://ps.oxfordjournals.org/ at University of North Dakota on June 24, 2015
at 30J hours of development. However, there was a significant difference in blastomere numbers at 40 and 41 hours of development, the greater number being found in the large-race rabbits. Byerly et ah, (1938), working with embryos from the Silkie, Rhode Island Red and reciprocal crosses between the two breeds, found no genetical difference in rate of cell division, cell size, or cell numbers. They did find genetically larger chicken embryos to be heavier. Egg size had more effect than genetic constitution for adult size on embryo size from 2-20 days' incubation. Consistent differences in rate of cell proliferation were found by Blunn and Gregory (1935) between White Leghorns and Rhode Island Reds. There was a correlation between cell number and embryo weight. The best measures of developmental rate were cell number, cell size, and rate of mitosis, rather than weight.
571
572
WILLIAM H. WILEY
TABLE 1.—Mean differences in egg weight, cell number, and cell size for 72-hour embryos M e a n cell number
M e a n egg weight Time No.
Large eggs
No.
Small eggs
M. D.
No.
Largeegg, No. origin
M e a n cell size
SmallM. D. egg, origin
No.
Largeegg origin
No.
Smaller? origin
M.D.
.01 mm. .01 m m .
grams
grams
Early Setting
19
54.8
27
46.7
8.1"
19
40.8
27
37.5
3.3**
19
2.63
27
2.90
0.27**
Late Setting
30
58.9
29
48.8
10.1**
30
41.8
29
38.8
3.0**
30
2.72
29
2.69
0.03
grams
0.1 m m .
" Highly significant.
Mean differences between 14-day embryos from large ^.nd from small eggs for cell number, cell size, and embryo weight are presented in Table 2. In the early setting of eggs, a mean difference of 2.43 + 1.38 in cell number favored the embryos from large eggs. In the late setting the large-egg embryos had 2.48+1.07 more cells per field than the embryos from small eggs. The latter difference was significant, but the former was not. There was no significant difference in size of individual cells between the two groups of embryos. The mean differences in embryo weight for the early and late setting were nonsignificant. In both tests the embryos from the large eggs were slightly heavier than the small-egg embryos. The differences were 0.20 + 0.60 gram in the early setting and 0.44 + 0.38 gram in the late setting. Table 3 presents the mean differences in cell number, cell size and embryo weight for 19-day embryos of the two groups in each setting of eggs. There was
TABLE 2.—Mean differences in cell number, cell size, and embryo weight for 14-day embryos M e a n cell size
M e a n cell number Time No.
Largeegg, origin
Early Setting
10
Late Setting
24
Largeegg origin
M e a n embryo weight Largeegg No. origin
Smallegg, M . D . origin
grams
grams grams
No.
M. D.
73.20
17
70.77
2.43
10
2.56
17
2.57
0.01
10
8.85
17
8.65
0.20
63.33
26
60.85
2.48*
24
2.50
26
2.55
0.55
24
9.31
26
8.87
0.44
No.
No.
M.D.
No.
.01 m m . .01 m m .
.01 m m .
* Significant.
Smallegg, origin
Smallegg, origin
Downloaded from http://ps.oxfordjournals.org/ at University of North Dakota on June 24, 2015
RESULTS
Mean differences in egg weight, cell number and cell size between the two egg-weight groups are recorded in Table 1 for the 72-hour embryos. The results of the late as well as the early setting of eggs are given. Weight differences of 8.1 + 0.82 grams in favor of the early setting of large eggs and 10.1+0.63 grams for the late setting of large eggs were highly significant. A cell number difference of 3.3 + 1.11 in the early setting of eggs was highly significant, the embryos from large eggs having the greater number of cells. There were 3.0 + 0.67 more cells per field in the embryos from large eggs of the late setting than in those from small eggs, and the mean difference was significant. Individual cells were 0.0027 + 0.0006 mm. larger in the embryos from small eggs than in those from large eggs in the early setting. This difference was highly significant. In the late setting, the difference in cell size of 0.0003 + 0.0002 mm. in favor of the embryos from large eggs was not significant.
573
INFLUENCE or EGG WEIGHT ON GROWTH RATE
TABLE 3.—Mean differences in cell number, cell size, and embryo weight for 19-day embryos Mean cell number Time No.
Mean cell size
Largeeg? No. origin
Smallegg origin
M. D.
No.
Largeegg origin
No.
Smallegg origin
Mean embryo weight M.D.
LargeNo. egg No. origin
Smallegf origin
M. D
grams
.01 m m . .01 mm.
grams
grams
Early Setting
10
65.5
6
53.33
12.17
10
2.88
6
3.01
0.23
10
19.5
6
21.83
2.33
Late Setting
25
59.0
25
59.04
0.04
25
2.67
25
2.78
0.11*
25
26.5
25
22.8
3.7"
.01 mm.
* Significant. _ ** Highly significant.
0.0011 ±0.0005 mm. in the late setting was significant. The weight difference of 2.33±2.24 grams in favor of the embryos from small eggs was not significant in the early setting. However,a highly significant difference of 3.7 ±0.91 grams favored the embryos from large eggs in the late setting. The latter difference was more reliable than the former, in that the sample in the early setting was extremely small. DISCUSSION
It was impossible to determine with any degree of accuracy the weight of the 72-hour embryos because the extra-embryonic membranes could not be removed to a uniform degree of cleanness. Therefore it was impossible to measure the differences in mean weight between the embryos from large and from small eggs at 72 hours' incubation. By the 14-day incubation period the embryos from large
counted for by the very small sample in this particular group and by errors in technique. Since the body weight difference reached a significant magnitude by the nineteenth day of incubation, but not the fourteenth day, it can be stated that the weight, and by inference the size, of the chick was limited significantly by the space in the egg shell during the last 2 or 3 days of incubation. The number of cells per field of embryonic tissue was controlled by egg weight, but the degree of control diminished as incubation progressed. The difference favored the embryos of large-egg origin in each setting of 72-hour and 14- • day embryos. The differences at 19 days' incubation were not significant. Only the difference in the late setting at 14 days' incubation was significant. The differences were highly significant in both settings of 72-hour embryos.
Downloaded from http://ps.oxfordjournals.org/ at University of North Dakota on June 24, 2015
eggs were heavier than those from small eggs, but the difference in mean embryo weight was not significant in either the early or the late setting of eggs. The body weights in the late setting of the 19-day embryos were greater for large-egg than for small-egg embryos. This difference was highly significant. In the early setting of 19-day embryos, the mean weight difference favored the embryos from small eggs, but the difference was not significant. This inconsistency may be ac-
a nonsignificant difference of 12.17 + 7.7 in mean cell number per field in favor of the early-set embryos from large eggs. In the late setting of eggs, there was no significant difference in cell number (0.04 ±2.06). The size of the cells in both settings was larger in the small-egg embryos than in the embryos from large eggs. In the early setting of eggs, the difference of 0.0023 + 0.0013 mm. in mean cell size was not significant; the difference of
574
WILLIAM H. WILEY
SUMMARY The pre-hatching effect of egg weight on growth rate was studied by comparing embryo weight, the number and size of cells in a given area of tissue in chick embryos from large and small eggs. In this study 248 embryos were sectioned for examination. Chick size was limited significantly by the space in the egg shell during the last 2 or 3 days of incubation. The number of cells per field of embryonic tissue was controlled by egg weight, but the degree of control diminished as incubation progressed. Cell size was negatively controlled by egg weight, more noticeably so in the earlier than in the later stages of embryonic development. ACKNOWLEDGEMENT The writer wishes to express his appreciation to the Arkansas Agricultural Experiment Station, Department of Animal Industry, for its cooperation in the be-
ginning phases of this study. Further, gratitude is expressed to Professor R. M. Sherwood, Professor C. B. Godbey, Dr. B. L. Warwick, Dr. J. H. Quisenberry, and Dr. G. E. Potter of the A. & M. College of Texas for their guidance and helpful suggestions. REFERENCES Benjamin, E. W., 1920. A study of selection for the size, shape, and color of hens' eggs. N. Y. Agr. Exp. Sta. Mim. 31. Blunn, C. T., and P. W. Gregory, 1935. The embryological basis of size inheritance in the chicken. J. Exp. Zool. 70: 397-414. Byerly, T. C , W. G. Helsel and J. P. Quinn, 1938. Growth in weight and cell number. Genetic effects in the chick embryo and chick. J. Exp. Zool. 78:185-203. Castle, W. E., 1939. A further study of size inheritance in rabbits, with special reference to the existance of genes for size characters. J. Exp. Zool. 53:421-454. Castle, W. E., and P. W. Gregory, 1929. The embryological basis of size inheritance in the rabbit. J. Morph. and Physiol. 48: 81-104. Castle, W. E., and P. W. Gregory, 1931. Further studies on the embryological basis of size inheritance in the rabbit. J. Exp. Zool. 59:199-211. Graham, J. C , 1931. Correlation of body weight at first egg to average monthly egg weights. Poultry Sci. 10:405. Halbersleben, D. L., and F. E. Mussehl, 1922. The relation of egg weight to chick weight a t hatching. Poultry Sci. 1: 143-144. Jull, M. A., and J. P. Quinn, 1925. The relationship between the weight of eggs and the weight of chicks according to sex. J. Agr. Res. 31: 223-226. Munroe, S. S., and I. L. Kosin, 1940. The existence of a sex difference in the weight of day old chicks, with further data on the egg-weight chick-weight relationship. Sci. Agr. 20: 586-591. Painter, T. S., 1928. Cell size and body size in rabbits. J. Exp. Zool. 50. Romanoff, A. L., 1929. Cycles in the prenatal growth of the domestic fowl. Science 70: 484. Upp, C. W., 1928. Egg weight, day old chick weight, and rate of growth in Single Comb Rhode Island Red Chicks. Poultry Sci. 7: 151-155.
Downloaded from http://ps.oxfordjournals.org/ at University of North Dakota on June 24, 2015
Cell size was controlled to a certain degree by egg weight. The control was in a negative direction, that is, the smaller the egg, the larger the size of individual cells in the tissues of the embryo. The magnitude of the differences in cell size was significant in early embryonic development. The degree of difference diminished as embryonic development progressed. The data reported here for the later stages of embryonic development are more nearly in accord with those of Blunn and Gregory (1935) than they are with the results of Painter (1928), Castle (1929), and Castle and Gregory (1931).
F
OR years it has been known that egg weight exerts a control on the weight of baby chicks at hatching time. A small egg produces a small chick, large egg produces a large chick. One would be apt to assume that such a start in life extends back into embryonic development prior to the hatching of the egg. Further, one might assume that any disadvantage suffered by the chick hatched from a small egg would be continued on through to maturity. Evidence was presented by Benjamin (1920), Hablersleben and Mussehl (1922), Upp (1928), Graham (1931), and Munroe and Kosin (1940) which showed a highly significant, positive relationship between egg weight and chick weight at hatching time. JuU and Quinn (1925) reported an influence of egg weight on body weight differences between male and female chicks, but contrasting results were published by Munroe and Kosin (1940) who 1 Contribution No. 745 of the Rhode Island Agricultural Experiment Station, Kingston, Rhode Island. 2 A condensation of a portion of a thesis submitted to the graduate faculty at the A. & M. College of Texas in partial fulfillment of the requirements for the degree of Doctor of Philosophy. 3 Formerly at the University of Arkansas, Department of Animal Industry, Fayetteville, Arkansas.
570
used percent of egg weight in expressing body weight of the chicks. One object of this study was to determine if possible, where egg weight exerted its first influence upon the rate of growth of the chick during embryonic development. Another objective was to determine the influence of egg weight on the size and number of cells in a given field of embryonic tissue. Though little work has been reported concerning eggs of different weights, considerable research has been carried out in the study of cell size and numbers as they affect the weight of individuals. According to Painter (1928), genetic factors are expressed early in life through the cell division differential. He began with segmenting rabbit eggs of the same size. There were more cells in larger embryos than in small ones, but there was no difference in nuclear size. Castle (1929) concurred with Painter when he gave the hereditary basis of large and small body size in rabbits as the differential rate of cell multiplication and growth. Castle and Gregory (1929) showed that the more rapid rate of cell multiplication in larger-race rabbits was transmitted equally by sperm and egg. Castle and Gregory (1931) found no significant difference in blastomere numbers between large- and small-race rabbits
Downloaded from http://ps.oxfordjournals.org/ at University of North Dakota on June 24, 2015
(Received for publication February 23, 1950)
INFLUENCE OF EGG WEIGHT ON GROWTH RATE
MATERIALS AND METHODS
This study was divided into two stages, an early setting (fall) and a late setting (spring). The stages of embryological development studied were 72 hours, 14 days, and 19 days of incubation. These periods were adopted for use since Romanoff (1929) considered them growth-constant periods. This work was started in November, 1946 and completed in February, 1948. Barred Plymouth Rock parent stocks were used which had been selected for the production of large eggs (57 grams or more) or for the production of small eggs (52 grams or less). The eggs used were collected each hour, marked and placed in a room cool enough to prevent continued cleavage division of cells. The eggs were weighed to the nearest 0.5 gram just prior
to setting. In an attempt to maintain an even oviduct-retention time, only eggs collected between 8:00 a.m. and 4:30 p.m. were set. During the incubation period, standard temperature and humidity were maintained. Paired samples of eggs were set up in such a way that two eggs, one large and one small, were set side by side at ten-minute intervals. This time interval permitted preparing and fixing each pair of embryos before the next pair was ready to be removed from the machine. Warm Bouin's fluid was used for fixing the embryos. Standard techniques were followed for dehydrating, clearing, and embedding the embryos in paraffin. Each 72-hour embryo was serially cross-sectioned at 7 n on a rotary microtome and placed on slides. The tissues were stained with Harris' haematoxylin and counterstained with eosin. Permanent slides were made of the desired portion of the embryos. Data were collected on the 72-hour embryos as to the number and the size of individual cells in a given area of the optic stalk. Determinations of cell numbers were made with an ocular disc micrometer divided into 0.1 mm. squares. Four adjacent squares were used as a field, and all cells were counted which had as much as one-half their nuclear area inside the outer margin of the squares. Ten fields were counted in each embryo. A magnification of 430X was used in the cell size and number determinations. The cell measurements were made with an adjustable ocular micrometer. In handling the 14-day and 19-day groups the dried embryos were weighed to the nearest 0.5 gram and only the left thigh was used for fixing. Cells were counted and measured in cross sections of the anterior end of the gastrocnemius muscle. All preparation was the same as that in the 72-hour group.
Downloaded from http://ps.oxfordjournals.org/ at University of North Dakota on June 24, 2015
at 30J hours of development. However, there was a significant difference in blastomere numbers at 40 and 41 hours of development, the greater number being found in the large-race rabbits. Byerly et ah, (1938), working with embryos from the Silkie, Rhode Island Red and reciprocal crosses between the two breeds, found no genetical difference in rate of cell division, cell size, or cell numbers. They did find genetically larger chicken embryos to be heavier. Egg size had more effect than genetic constitution for adult size on embryo size from 2-20 days' incubation. Consistent differences in rate of cell proliferation were found by Blunn and Gregory (1935) between White Leghorns and Rhode Island Reds. There was a correlation between cell number and embryo weight. The best measures of developmental rate were cell number, cell size, and rate of mitosis, rather than weight.
571
572
WILLIAM H. WILEY
TABLE 1.—Mean differences in egg weight, cell number, and cell size for 72-hour embryos M e a n cell number
M e a n egg weight Time No.
Large eggs
No.
Small eggs
M. D.
No.
Largeegg, No. origin
M e a n cell size
SmallM. D. egg, origin
No.
Largeegg origin
No.
Smaller? origin
M.D.
.01 mm. .01 m m .
grams
grams
Early Setting
19
54.8
27
46.7
8.1"
19
40.8
27
37.5
3.3**
19
2.63
27
2.90
0.27**
Late Setting
30
58.9
29
48.8
10.1**
30
41.8
29
38.8
3.0**
30
2.72
29
2.69
0.03
grams
0.1 m m .
" Highly significant.
Mean differences between 14-day embryos from large ^.nd from small eggs for cell number, cell size, and embryo weight are presented in Table 2. In the early setting of eggs, a mean difference of 2.43 + 1.38 in cell number favored the embryos from large eggs. In the late setting the large-egg embryos had 2.48+1.07 more cells per field than the embryos from small eggs. The latter difference was significant, but the former was not. There was no significant difference in size of individual cells between the two groups of embryos. The mean differences in embryo weight for the early and late setting were nonsignificant. In both tests the embryos from the large eggs were slightly heavier than the small-egg embryos. The differences were 0.20 + 0.60 gram in the early setting and 0.44 + 0.38 gram in the late setting. Table 3 presents the mean differences in cell number, cell size and embryo weight for 19-day embryos of the two groups in each setting of eggs. There was
TABLE 2.—Mean differences in cell number, cell size, and embryo weight for 14-day embryos M e a n cell size
M e a n cell number Time No.
Largeegg, origin
Early Setting
10
Late Setting
24
Largeegg origin
M e a n embryo weight Largeegg No. origin
Smallegg, M . D . origin
grams
grams grams
No.
M. D.
73.20
17
70.77
2.43
10
2.56
17
2.57
0.01
10
8.85
17
8.65
0.20
63.33
26
60.85
2.48*
24
2.50
26
2.55
0.55
24
9.31
26
8.87
0.44
No.
No.
M.D.
No.
.01 m m . .01 m m .
.01 m m .
* Significant.
Smallegg, origin
Smallegg, origin
Downloaded from http://ps.oxfordjournals.org/ at University of North Dakota on June 24, 2015
RESULTS
Mean differences in egg weight, cell number and cell size between the two egg-weight groups are recorded in Table 1 for the 72-hour embryos. The results of the late as well as the early setting of eggs are given. Weight differences of 8.1 + 0.82 grams in favor of the early setting of large eggs and 10.1+0.63 grams for the late setting of large eggs were highly significant. A cell number difference of 3.3 + 1.11 in the early setting of eggs was highly significant, the embryos from large eggs having the greater number of cells. There were 3.0 + 0.67 more cells per field in the embryos from large eggs of the late setting than in those from small eggs, and the mean difference was significant. Individual cells were 0.0027 + 0.0006 mm. larger in the embryos from small eggs than in those from large eggs in the early setting. This difference was highly significant. In the late setting, the difference in cell size of 0.0003 + 0.0002 mm. in favor of the embryos from large eggs was not significant.
573
INFLUENCE or EGG WEIGHT ON GROWTH RATE
TABLE 3.—Mean differences in cell number, cell size, and embryo weight for 19-day embryos Mean cell number Time No.
Mean cell size
Largeeg? No. origin
Smallegg origin
M. D.
No.
Largeegg origin
No.
Smallegg origin
Mean embryo weight M.D.
LargeNo. egg No. origin
Smallegf origin
M. D
grams
.01 m m . .01 mm.
grams
grams
Early Setting
10
65.5
6
53.33
12.17
10
2.88
6
3.01
0.23
10
19.5
6
21.83
2.33
Late Setting
25
59.0
25
59.04
0.04
25
2.67
25
2.78
0.11*
25
26.5
25
22.8
3.7"
.01 mm.
* Significant. _ ** Highly significant.
0.0011 ±0.0005 mm. in the late setting was significant. The weight difference of 2.33±2.24 grams in favor of the embryos from small eggs was not significant in the early setting. However,a highly significant difference of 3.7 ±0.91 grams favored the embryos from large eggs in the late setting. The latter difference was more reliable than the former, in that the sample in the early setting was extremely small. DISCUSSION
It was impossible to determine with any degree of accuracy the weight of the 72-hour embryos because the extra-embryonic membranes could not be removed to a uniform degree of cleanness. Therefore it was impossible to measure the differences in mean weight between the embryos from large and from small eggs at 72 hours' incubation. By the 14-day incubation period the embryos from large
counted for by the very small sample in this particular group and by errors in technique. Since the body weight difference reached a significant magnitude by the nineteenth day of incubation, but not the fourteenth day, it can be stated that the weight, and by inference the size, of the chick was limited significantly by the space in the egg shell during the last 2 or 3 days of incubation. The number of cells per field of embryonic tissue was controlled by egg weight, but the degree of control diminished as incubation progressed. The difference favored the embryos of large-egg origin in each setting of 72-hour and 14- • day embryos. The differences at 19 days' incubation were not significant. Only the difference in the late setting at 14 days' incubation was significant. The differences were highly significant in both settings of 72-hour embryos.
Downloaded from http://ps.oxfordjournals.org/ at University of North Dakota on June 24, 2015
eggs were heavier than those from small eggs, but the difference in mean embryo weight was not significant in either the early or the late setting of eggs. The body weights in the late setting of the 19-day embryos were greater for large-egg than for small-egg embryos. This difference was highly significant. In the early setting of 19-day embryos, the mean weight difference favored the embryos from small eggs, but the difference was not significant. This inconsistency may be ac-
a nonsignificant difference of 12.17 + 7.7 in mean cell number per field in favor of the early-set embryos from large eggs. In the late setting of eggs, there was no significant difference in cell number (0.04 ±2.06). The size of the cells in both settings was larger in the small-egg embryos than in the embryos from large eggs. In the early setting of eggs, the difference of 0.0023 + 0.0013 mm. in mean cell size was not significant; the difference of
574
WILLIAM H. WILEY
SUMMARY The pre-hatching effect of egg weight on growth rate was studied by comparing embryo weight, the number and size of cells in a given area of tissue in chick embryos from large and small eggs. In this study 248 embryos were sectioned for examination. Chick size was limited significantly by the space in the egg shell during the last 2 or 3 days of incubation. The number of cells per field of embryonic tissue was controlled by egg weight, but the degree of control diminished as incubation progressed. Cell size was negatively controlled by egg weight, more noticeably so in the earlier than in the later stages of embryonic development. ACKNOWLEDGEMENT The writer wishes to express his appreciation to the Arkansas Agricultural Experiment Station, Department of Animal Industry, for its cooperation in the be-
ginning phases of this study. Further, gratitude is expressed to Professor R. M. Sherwood, Professor C. B. Godbey, Dr. B. L. Warwick, Dr. J. H. Quisenberry, and Dr. G. E. Potter of the A. & M. College of Texas for their guidance and helpful suggestions. REFERENCES Benjamin, E. W., 1920. A study of selection for the size, shape, and color of hens' eggs. N. Y. Agr. Exp. Sta. Mim. 31. Blunn, C. T., and P. W. Gregory, 1935. The embryological basis of size inheritance in the chicken. J. Exp. Zool. 70: 397-414. Byerly, T. C , W. G. Helsel and J. P. Quinn, 1938. Growth in weight and cell number. Genetic effects in the chick embryo and chick. J. Exp. Zool. 78:185-203. Castle, W. E., 1939. A further study of size inheritance in rabbits, with special reference to the existance of genes for size characters. J. Exp. Zool. 53:421-454. Castle, W. E., and P. W. Gregory, 1929. The embryological basis of size inheritance in the rabbit. J. Morph. and Physiol. 48: 81-104. Castle, W. E., and P. W. Gregory, 1931. Further studies on the embryological basis of size inheritance in the rabbit. J. Exp. Zool. 59:199-211. Graham, J. C , 1931. Correlation of body weight at first egg to average monthly egg weights. Poultry Sci. 10:405. Halbersleben, D. L., and F. E. Mussehl, 1922. The relation of egg weight to chick weight a t hatching. Poultry Sci. 1: 143-144. Jull, M. A., and J. P. Quinn, 1925. The relationship between the weight of eggs and the weight of chicks according to sex. J. Agr. Res. 31: 223-226. Munroe, S. S., and I. L. Kosin, 1940. The existence of a sex difference in the weight of day old chicks, with further data on the egg-weight chick-weight relationship. Sci. Agr. 20: 586-591. Painter, T. S., 1928. Cell size and body size in rabbits. J. Exp. Zool. 50. Romanoff, A. L., 1929. Cycles in the prenatal growth of the domestic fowl. Science 70: 484. Upp, C. W., 1928. Egg weight, day old chick weight, and rate of growth in Single Comb Rhode Island Red Chicks. Poultry Sci. 7: 151-155.
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Cell size was controlled to a certain degree by egg weight. The control was in a negative direction, that is, the smaller the egg, the larger the size of individual cells in the tissues of the embryo. The magnitude of the differences in cell size was significant in early embryonic development. The degree of difference diminished as embryonic development progressed. The data reported here for the later stages of embryonic development are more nearly in accord with those of Blunn and Gregory (1935) than they are with the results of Painter (1928), Castle (1929), and Castle and Gregory (1931).