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Variation in the Development of Embryos of Hens’ Eggs
Variation in the Development of Embryos of Hens' Eggs E. H. MCNALLY AND T. C. BYERLY National Agricultural Research Center, Beltsville, Maryland (Presented at Annual Meeting, August 6-9, 1935)
T
HE embryonic chick at the time of the laying of the eggs is usually subjected to the most extreme variations in environment it will have to withstand before hatching. In cold weather it may be quickly chilled, and during hot weather it may continue to develop, without pausing, for several hours before being stored under conditions that stop development while being held before incubation. Patterson (1910) found that the embryos show variation in development at the time of laying and Hutt and Pilkey (1930) have pointed out that some embryonic stages may be especially sensitive to external conditions, which may cause abnormal development or death, but would not affect the embryo at other stages. Hays and Nicolaides (1934) considered the variation, in embryonic development of fresh-laid eggs, among hens to be of primary importance, and stated that high hatching hens laid fertile eggs with more fully developed embryos than low hatching hens. No attempts have been made by former workers to observe the factors that may affect the variation in embryonic development in eggs from individual hens. Perhaps this is partly owing to the fact that the embryo at this stage does not show a character suitable for quantitative measurements, and that the embryo is very difficult to prepare for examination in a large enough number for comparative studies. EXPERIMENTAL WORK In order to obtain a character suitable for quantitative measurements, and at the
same time obtain an embryo at a size suitable for manipulation, all of the eggs were incubated for 48 hours. During November, 1934, eggs were collected from the trapnests of two pens of Single Comb White Leghorn pullets at each hour from 8 a.m. to 4 p.m., inclusive, and the time of laying of each egg recorded at the Beltsville Research Center, Beltsville, Maryland. The birds were confined in the poultry house with a mash always available. The eggs were immediately placed in the incubator from the trapnest. This was done to reduce holding and cooling effects to a minimum. The eggs were incubated in a forced-draft electric incubator at 99 ^ ° F . and 60 percent relative humidity for exactly 48 hours, and then removed from the incubator and opened. The embryos were quickly removed from the yolks and the somites of each embryo were counted under a binocular microscope. In all, 640 embryos were treated in this way. In order to make comparisons of embryonic development and hatchability, hatchability data on 1,400 eggs from the same hens were collected during December, 1934; trapnesting each hour and then holding the eggs at room temperature for about three weeks in order to produce aging of the embryos and consequent variation in hatchability. Later, in May, 1935, 315 eggs were collected from part of the original flocks, under the same conditions. Egg weights were obtained and the eggs were opened after 48 hours' incubation for determination of the number of somites.
[280]
JULY,
1936.
VOL.
XV,
No.
RESULTS
The data collected were classified from several points of view to show the effect of various factors. A variation of from nine to 26 somites after 48 hours of incubation was found, with an average of 19.18 somites for all embryos observed. The average number of somites of embryos from eggs laid at various hours during the day was determined and plotted (Figure 1). A rapid decrease in average somite number is observed from 8 to 9 a.m., with a slower decrease during the rest of the day, except for a small increase about 1 p.m. This confirms Hutt and Pilkey's (1930) deductions that the embryos in morning eggs (before 9 a.m.) should show a greater development than that of eggs laid later. The embryos were classified according to the time interval between successive eggs of a clutch and the average somite number of the embryos computed (Figure 2). An increase in average number of somites with an
\. \ \
281
4
^
TIME OF DAY (HOURS)
Fig. 1. 0 = Relation between development after 48 hours' incubation and time of day egg was laid. X = Relation between percent hatch and time of day egg was laid.
increase in the number of hours between eggs takes place from 24 to 27 hours with a slight decrease after 27 hours. The decrease in average somite number in embryos from eggs over the 27-hour interval depends to a large extent upon the eggs in two-egg clutches, since the larger number of all eggs with 27 hours or more
between eggs are the second egg of a twoegg clutch. In separating the somite number of embryos from these eggs into class intervals an unequal distribution is observed, which forms two classes that overlap and give a bimodal curve. Part of the embryos show an approximate true 28- or 29-hour development with a small number showing
£50 ±40 <30 20 10 24
25 26 INTERVAL
27 28 BETWEEN EGGS
29 30 (HOURS)
31
Fig. 2. O = Relation between development after 48 hours' incubation and time interval between laying of eggs. X = Relation between percent hatch and time interval between laying of eggs.
less development bringing the average down. Two-egg clutches as they appear on the trapnest cards, may be divided into a true two-egg clutch, and two eggs laid in rapid succession with an interval between the time the egg is laid and ovulation takes place. Atwood and Weakley (1917) and Jull (1924) have stated that the second egg of a two-egg clutch weighs less than the first. However, egg weights of the second egg of a two-egg clutch when placed in class intervals, show a similar bimodal distribution to that of somite number. The similarity in bimodal distribution of egg weight and somite number of the second egg ih two-egg clutches led to a collection of data on somite number and egg weight. It was necessary to eliminate a large amount of individual variation by plotting deviations from the average for each hen of both egg weight and somite number of the embryos. These have been plotted (Figure 3)
282
POULTRY
and show an increase in somite number with an increase in egg weight for most eggs, with a tendency for the smaller eggs to hav embryos developed farther than would be predicted from their weight. The data on hatchability were classified in much the same manner. Fifty percent of all fertile eggs set hatched. When time of day when the eggs were laid is considered (Figure 1) a slight in+4 UJ
o ~ ui m
**
+2
•f z "-III
+1
-!•*-
o r
0
-1 >S: UJ O
SCIENCE
DISCUSSION
Although Patterson (1910) stated that ovulation immediately followed laying, he had only a small number of data. Warren and Scott (1934) have also gathered data on this point. We have also attempted to make observations on time of ovulation. Time of laying was observed and then half an hour later the hens were sacrificed. Of 32 hens sacrificed 20 had an ovum in the oviduct. Although, according to the laying cycle of the hens, they should have laid on the next day it is possible that some would not have laid. At least we are justified in assuming a high correlation between the time of laying of an egg and immediate ovulation. After ovulation the egg weight
Or -2
00
K i -3
90
-4 - 5 - 4 -3 - 2 -I 0 +1 +2 -1-3 + 4 +5 + 6 AVERAGE DEVIATION FROM AVERAGE EGG WEIGHT (GRAMS)
FIG. 3. Relation between average deviation from average somite number at 48 hours and average deviation from average egg weight.
crease in hatchability is noted in eggs laid later in the day. This would confirm the work of Funk (1934) but differs from that of Hutt and Pilkey (1930). If the time interval between successive eggs of a clutch is considered with percentage hatchability (Figure 2), an increase in hatchability is observed from the 24 to 27 hour interval with a rapid decrease in hatchability in those eggs over 2 7 hours between successive eggs. Finally, it was possible to compare the percentage hatchability with the average number of somites for the same hens (Figure 4). An increase in hatchability is observed in hens laying eggs giving an average somite number up to 19 or 20 somites, with a rapid decrease in hatchability in hens laying eggs averaging greater than the latter number.
80
^
70 zIII o 60 a. UJ a. 5 0
V-
X
o 40 14T
40 20 10 0
14
15
16
17 18 19 2 0 21 SOMITES (AVERAGE)
22
23
FIG. 4. Relation between percent hatch and development after 48 hours' incubation.
and embryonic development for eggs from that particular hen depend upon the length of time before the egg is laid. This, in turn, depends upon the length of clutch in which that egg is laid as Atwood (1929) has shown. The length of time the eggs are held during formation also affects the hatchability of the eggs. If the egg is only 24 or 25 hours in the oviduct, the embryo is too
JULY,
1936.
VOL.
XV,
No.
283
4
young to stand the variations in environment it must undergo. If the egg is held over 27 hours its hatchability is reduced. The high body temperature of the hen may have a deteriorating effect after a certain stage in development, injuring the embryos as 105° or 106°F. would during incubation. SUMMARY
1. Embryos from fresh, fertile eggs after being incubated for 48 hours show a variation in development of from nine to 26 somites. 2. The average number of somites of embryos increases with the length of time between eggs of a clutch from 24 to 27 hours. 3. Maximum hatchability was found in eggs having a length of time between successive eggs of 27 hours, with a decrease in hatchability when the length of time is more or less than 27 hours. 4. In eggs from individual hens, the num-
ber of somites increases with egg weight when eggs weigh above a minimum value for the hen at that time. 5. In 32 hens sacrificed, 20 were found to have ovulated within half an hour after laying. BIBLIOGRAPHY
Atwood, H. and Weakley, C. E., 1917. Certain characters of hen eggs. W. Va. Agr. Exp. Sta. Bull. 166. , 1929. Observations concerning the time factor in egg production. Poul. Sci. 8:137-140. Funk, E. M., 1934. Relation of time of laying to hatchability. Poul. Sci. 8:184-187. Hays, F. A. and Nicolaides, Costas, 1934. Variability in development of fresh-laid hens' eggs. Poul. Sci. 13:74. Hutt, F. B. and Pilkey, A. M., 1930. Studies in embryonic mortality in the fowl. Poul. Sci. 9:194-203. Jull, M. A., 1924. Egg weight in relation to production. Poul. Sci. 3:77. Patterson, J. T., 1910. Studies in the early development of the hen's egg. Jour. Morph. 21:101-134. Warren, D. C. and Scott, H. M., 1934. Ovulation in the domestic hen. Science, 80:461-62.
T
HE embryonic chick at the time of the laying of the eggs is usually subjected to the most extreme variations in environment it will have to withstand before hatching. In cold weather it may be quickly chilled, and during hot weather it may continue to develop, without pausing, for several hours before being stored under conditions that stop development while being held before incubation. Patterson (1910) found that the embryos show variation in development at the time of laying and Hutt and Pilkey (1930) have pointed out that some embryonic stages may be especially sensitive to external conditions, which may cause abnormal development or death, but would not affect the embryo at other stages. Hays and Nicolaides (1934) considered the variation, in embryonic development of fresh-laid eggs, among hens to be of primary importance, and stated that high hatching hens laid fertile eggs with more fully developed embryos than low hatching hens. No attempts have been made by former workers to observe the factors that may affect the variation in embryonic development in eggs from individual hens. Perhaps this is partly owing to the fact that the embryo at this stage does not show a character suitable for quantitative measurements, and that the embryo is very difficult to prepare for examination in a large enough number for comparative studies. EXPERIMENTAL WORK In order to obtain a character suitable for quantitative measurements, and at the
same time obtain an embryo at a size suitable for manipulation, all of the eggs were incubated for 48 hours. During November, 1934, eggs were collected from the trapnests of two pens of Single Comb White Leghorn pullets at each hour from 8 a.m. to 4 p.m., inclusive, and the time of laying of each egg recorded at the Beltsville Research Center, Beltsville, Maryland. The birds were confined in the poultry house with a mash always available. The eggs were immediately placed in the incubator from the trapnest. This was done to reduce holding and cooling effects to a minimum. The eggs were incubated in a forced-draft electric incubator at 99 ^ ° F . and 60 percent relative humidity for exactly 48 hours, and then removed from the incubator and opened. The embryos were quickly removed from the yolks and the somites of each embryo were counted under a binocular microscope. In all, 640 embryos were treated in this way. In order to make comparisons of embryonic development and hatchability, hatchability data on 1,400 eggs from the same hens were collected during December, 1934; trapnesting each hour and then holding the eggs at room temperature for about three weeks in order to produce aging of the embryos and consequent variation in hatchability. Later, in May, 1935, 315 eggs were collected from part of the original flocks, under the same conditions. Egg weights were obtained and the eggs were opened after 48 hours' incubation for determination of the number of somites.
[280]
JULY,
1936.
VOL.
XV,
No.
RESULTS
The data collected were classified from several points of view to show the effect of various factors. A variation of from nine to 26 somites after 48 hours of incubation was found, with an average of 19.18 somites for all embryos observed. The average number of somites of embryos from eggs laid at various hours during the day was determined and plotted (Figure 1). A rapid decrease in average somite number is observed from 8 to 9 a.m., with a slower decrease during the rest of the day, except for a small increase about 1 p.m. This confirms Hutt and Pilkey's (1930) deductions that the embryos in morning eggs (before 9 a.m.) should show a greater development than that of eggs laid later. The embryos were classified according to the time interval between successive eggs of a clutch and the average somite number of the embryos computed (Figure 2). An increase in average number of somites with an
\. \ \
281
4
^
TIME OF DAY (HOURS)
Fig. 1. 0 = Relation between development after 48 hours' incubation and time of day egg was laid. X = Relation between percent hatch and time of day egg was laid.
increase in the number of hours between eggs takes place from 24 to 27 hours with a slight decrease after 27 hours. The decrease in average somite number in embryos from eggs over the 27-hour interval depends to a large extent upon the eggs in two-egg clutches, since the larger number of all eggs with 27 hours or more
between eggs are the second egg of a twoegg clutch. In separating the somite number of embryos from these eggs into class intervals an unequal distribution is observed, which forms two classes that overlap and give a bimodal curve. Part of the embryos show an approximate true 28- or 29-hour development with a small number showing
£50 ±40 <30 20 10 24
25 26 INTERVAL
27 28 BETWEEN EGGS
29 30 (HOURS)
31
Fig. 2. O = Relation between development after 48 hours' incubation and time interval between laying of eggs. X = Relation between percent hatch and time interval between laying of eggs.
less development bringing the average down. Two-egg clutches as they appear on the trapnest cards, may be divided into a true two-egg clutch, and two eggs laid in rapid succession with an interval between the time the egg is laid and ovulation takes place. Atwood and Weakley (1917) and Jull (1924) have stated that the second egg of a two-egg clutch weighs less than the first. However, egg weights of the second egg of a two-egg clutch when placed in class intervals, show a similar bimodal distribution to that of somite number. The similarity in bimodal distribution of egg weight and somite number of the second egg ih two-egg clutches led to a collection of data on somite number and egg weight. It was necessary to eliminate a large amount of individual variation by plotting deviations from the average for each hen of both egg weight and somite number of the embryos. These have been plotted (Figure 3)
282
POULTRY
and show an increase in somite number with an increase in egg weight for most eggs, with a tendency for the smaller eggs to hav embryos developed farther than would be predicted from their weight. The data on hatchability were classified in much the same manner. Fifty percent of all fertile eggs set hatched. When time of day when the eggs were laid is considered (Figure 1) a slight in+4 UJ
o ~ ui m
**
+2
•f z "-III
+1
-!•*-
o r
0
-1 >S: UJ O
SCIENCE
DISCUSSION
Although Patterson (1910) stated that ovulation immediately followed laying, he had only a small number of data. Warren and Scott (1934) have also gathered data on this point. We have also attempted to make observations on time of ovulation. Time of laying was observed and then half an hour later the hens were sacrificed. Of 32 hens sacrificed 20 had an ovum in the oviduct. Although, according to the laying cycle of the hens, they should have laid on the next day it is possible that some would not have laid. At least we are justified in assuming a high correlation between the time of laying of an egg and immediate ovulation. After ovulation the egg weight
Or -2
00
K i -3
90
-4 - 5 - 4 -3 - 2 -I 0 +1 +2 -1-3 + 4 +5 + 6 AVERAGE DEVIATION FROM AVERAGE EGG WEIGHT (GRAMS)
FIG. 3. Relation between average deviation from average somite number at 48 hours and average deviation from average egg weight.
crease in hatchability is noted in eggs laid later in the day. This would confirm the work of Funk (1934) but differs from that of Hutt and Pilkey (1930). If the time interval between successive eggs of a clutch is considered with percentage hatchability (Figure 2), an increase in hatchability is observed from the 24 to 27 hour interval with a rapid decrease in hatchability in those eggs over 2 7 hours between successive eggs. Finally, it was possible to compare the percentage hatchability with the average number of somites for the same hens (Figure 4). An increase in hatchability is observed in hens laying eggs giving an average somite number up to 19 or 20 somites, with a rapid decrease in hatchability in hens laying eggs averaging greater than the latter number.
80
^
70 zIII o 60 a. UJ a. 5 0
V-
X
o 40 14T
40 20 10 0
14
15
16
17 18 19 2 0 21 SOMITES (AVERAGE)
22
23
FIG. 4. Relation between percent hatch and development after 48 hours' incubation.
and embryonic development for eggs from that particular hen depend upon the length of time before the egg is laid. This, in turn, depends upon the length of clutch in which that egg is laid as Atwood (1929) has shown. The length of time the eggs are held during formation also affects the hatchability of the eggs. If the egg is only 24 or 25 hours in the oviduct, the embryo is too
JULY,
1936.
VOL.
XV,
No.
283
4
young to stand the variations in environment it must undergo. If the egg is held over 27 hours its hatchability is reduced. The high body temperature of the hen may have a deteriorating effect after a certain stage in development, injuring the embryos as 105° or 106°F. would during incubation. SUMMARY
1. Embryos from fresh, fertile eggs after being incubated for 48 hours show a variation in development of from nine to 26 somites. 2. The average number of somites of embryos increases with the length of time between eggs of a clutch from 24 to 27 hours. 3. Maximum hatchability was found in eggs having a length of time between successive eggs of 27 hours, with a decrease in hatchability when the length of time is more or less than 27 hours. 4. In eggs from individual hens, the num-
ber of somites increases with egg weight when eggs weigh above a minimum value for the hen at that time. 5. In 32 hens sacrificed, 20 were found to have ovulated within half an hour after laying. BIBLIOGRAPHY
Atwood, H. and Weakley, C. E., 1917. Certain characters of hen eggs. W. Va. Agr. Exp. Sta. Bull. 166. , 1929. Observations concerning the time factor in egg production. Poul. Sci. 8:137-140. Funk, E. M., 1934. Relation of time of laying to hatchability. Poul. Sci. 8:184-187. Hays, F. A. and Nicolaides, Costas, 1934. Variability in development of fresh-laid hens' eggs. Poul. Sci. 13:74. Hutt, F. B. and Pilkey, A. M., 1930. Studies in embryonic mortality in the fowl. Poul. Sci. 9:194-203. Jull, M. A., 1924. Egg weight in relation to production. Poul. Sci. 3:77. Patterson, J. T., 1910. Studies in the early development of the hen's egg. Jour. Morph. 21:101-134. Warren, D. C. and Scott, H. M., 1934. Ovulation in the domestic hen. Science, 80:461-62.