636.5

C3A5f

FACTORS INFLUENCING THE SIZE, SHAPE
AND PHYSICAL CONSTITUTION OF THE
EGG OF THE DOMESTIC FOWL

BY

MAYNIE R. CURTIS

AN ABSTRACT OF A THESIS SUBMITTED IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF
PHILOSOPHY IN THE UNIVERSITY OF MICHIGAN

ORONO, MAINE

1914

636.5

C3456

LIBRARY

OF

THE UNIVERSITY OF TEXAS

PRESENTED BY

31 33-91 4-5M 6449

BULLETIN 228.

FACTORS INFLUENCING THE SIZE, SHAPE AND
PHYSICAL CONSTITUTION OF THE EGG

OF THE DOMESTIC FOWL*

: : V; ; > ■

* O 3 ^ J 3 » > O 9 *

1 ° . ’ % * ' *

Maynie: R. Curtis.

3 -> 3 3

In a study 'pt the physiology of reproduction in the domestic

fowl one miist soon recognize the fact that the number of eggs
produced is only a rough measure of the reproductive activity
for the eggs produced are not all equivalent. The complicated
physiological processes involved in the production of an egg
are so influenced by both heredity and environment that. they
result in quite unequal products. Eggs differ greatly in every
character. The scientific and economic importance of differ-
ences in size and quality is obvious.

Investigations on inheritance in poultry have been in progress
at this laboratory for several years. In this work tue eggs oi
about two hundred hens of one breed are handled individually
each season. The variation in these eggs is very great. The
four following facts in regard to it are apparent.

i. The eggs of different individuals of the same strain vary
in size, shape, color and markings.

* This paper is an abstract, setting forth the more important results
and conclusions, of an extended paper by the same author, published
under the following title: “A Biometrical Study of Egg Production in
the Domestic Fpwl. IV. Factors Influencing the Size, Shape and
Physical Constitution of Eggs,” in Archiv fuer Entwicklungsmechanik
der Organism (Roux). Bd 39, pp. 217, 1914. The three previous
papers in the series are :

Pearl, R. and ^Surface, F. M. A Biometrical Study of Egg Produc-
tion in the Domestic Fowl. I. Variation in Annual Egg Production,
U. S. Department of Agr., Bur. of Animal Industry, Bui. no, pp. 1-80,
1909. II. Seasonal Distribution of Egg Production. Ibid. pp. 81-170,
1911. III. Variation and Correlation in the Physical Characters of
the Egg. Ibid. pp. 171 — , 1914-

8902b

Io6 MAINE AGRICULTURAL EXPERIMENT STATION. I9I4.

2. The eggs of an individual are more like each other than
they are like the eggs of other individuals of the same strain.

3. In spite of this resemlblance between the eggs of the
individual they nevertheless show a certain degree of variation
in all their characters.

4. The first eggs laid by the pullets are smaller than those
laid by the same birds later.

An analysis of the factors underlying this individuality and
variation in the quantitative characters of the egg offers a new
point of attack in the study of -the physiology of egg produc-
tion. A statistical study wasrnmde of the: quantitative charac-
ters of all of the eggs laid by twenty-two ’Barred Plymouth
Rock birds during theiTtfi^st two laying years.- ./These birds
were all from the Maine Agricultural E^rpenmeni Station
strain, the purity of which has been amply proven by breeding
experiments.

This paper presents briefly the results of this investigation
of the shape and size, both absolute and relative, and the pro-
portion of parts of the successive eggs of each of these birds.
It includes (a) a study of the individuality of the eggs of each
bird and the variation among them; (b) a study of the relation
of the individuality of the eggs to other facts known about the
bird, such as body weight, relative size of the several visceral
organs, age, number of eggs laid, and tendency to broodiness ;
(c) the variation in egg characters in relation to the age of the
bird, the seasonal reproductive cycles and the intensity of the
reproductive activity, including the grouping of the successive
eggs into clutches and litters, the size of these groups and the
position of the eggs within them.

The Individuality oe the Eggs oe Each Bird in Regard to
Shape, Size, and Size and Proportion of Parts.

It was not necessary to resort to mathematical calculation
to demonstrate the fact that the eggs of the different individual
birds used in this investigation varied considerably in respect
to size and shape. This inter-dndividual variation was easily
seen by comparing the eggs of the different individuals. The
eggs of one bird were from one-fourth to one-third larger than

SIZE, SHAPE AND PHYSICAL CONSTITUTION OF THE EGG. 107

the eggs of one of the others. Some of the birds laid eggs
which were long and narrow/others those which were short and
broad. The eggs of some individuals were distinctly pointed,
while those of others were not.

Not only was it possible to see this individuality by compai-
ing the eggs of the different individuals laid at the same sea-
son, but by preserving some of the shells of the eggs of each
bird at various seasons it was possible to see that the relative
size and the shape of the eggs of a fowl are to a large extent
permanent characteristics of the individual.

The present investigation deals primarily with the quantita-

0 or measurable characters of the egg, but that the individ-
uality of the different birds manifests itself also in certain
qualitative characters is illustrated by a comparison of the coloi
and markings of the eggs of the different birds.

This variation which is seen by a comparison of the egg a
themselves is shown equally clearly by a comparison of the
measurements of the characters. It is not confined to the ex-
ternally visible characters (dimensions, shape and size) but
extends to the size and the proportion of the parts which make
up the egg.

A statistical analysis of the data shows the following points :

INT^R-INDIVIDUAL VARIATION.

1. The characteristic eggs of the different individuals show
variation in all egg characters.

2. They differ more in size than in shape, and they differ
more in the amount of albumen and shell than in the size of the
yolk.

3. There is a general correlation between the various char-
acters of the egg. That is, when a bird’s eggs are large they
are both long and broad and contain a large yolk with a large
amount of albumen and shell. The amount of yolk, however, is
not proportionately large, so that the birds which lay large
eggs lay eggs which have a relatively small percentage of yolk.

4. In spite of this general relationship the eggs of one in-
dividual may vary disproportionately in any one of the char-
acters measured.

5. There is an evident individuality among the birds also
in the amount of variation in the egg characters. The uniform

108 MAINE AGRICULTURAL EXPERIMENT STATION. I9I4.

ity (or variability) of the egg characters of an individual tends
to be general, but may manifest itself in different characters to
quite different degrees.

6. A comparison of the relative degree of variation in the
individual and in the race shows that in general the eggs of
an individual resemble each other much more closely than do
a random sample of the eggs of the same strain. The eggs of
certain individuals, however, show a much higher percentage of
the variation of the race than those of some of the other indi-
viduals. In certain egg characters there is a general tendency
for the individual variation to approach more nearly the limits
of the race variation than in other characters. Finally certain
characters may display a variation relatively greater in a par-
ticular individual than in the race. This indicates that the
egg characters are determined by an interaction of hereditary
and environmental forces which may cause a particular char-
acter to reach or surpass the racial limits of variation within the
eggs of one individual.

INTRA-INDIVIDUAL VARIATION.

The different degree of variation shown by the several egg
characters in each individual is also clearly seem. The four fol-
lowing relationships between the degree of variation in the
separate characters are clearly demonstrated, either by compar-
ing the average individual variation for each character taking
the group of birds as a whole, or by considering separately the
several coefficients for each bird.

1. The egg varies more in respect to weight of shell and
weight of yolk than in respect to any of the other characters .
A few individuals show significant differences between the
amount of variation in these characters. The eggs of some
individuals vary more in respect to the one and some in respect
to the other.

2. Egg weight and albumen weight are about equally varia-
ble. They are decidedly less variable than shell weight and
yolk weight and much more variable than either length oh
breadth. The difference between the variation for these two
characters is significant for very few individuals.

3. Both length and breadth are much less variable than any
of the weight characters of the egg.

SIZE, SHAPE AND PHYSICAL CONSTITUTION OF THE EGG. 109

4. The egg is more variable in length than in breadth. In a
few individuals the difference between the variation in these
characters is not significant.

'The following list arranges the egg characters in the order
of their variability.

j Egg weight.

| Yolk weight.

( Albumen weight.

| Shell weight.

Length.

Breadth.

It is especially worthy of note than the variation coefficients
for yolk and shell are much higher than for the whole egg,
and the coefficient for albumen is at least as high. There
must then be a compensatory variation among the parts. The
data show also that the eggs of the breeding season have dis-
proportionately large yolks and a consequent smaller percentage
of albumen than the eggs at other seasons.

The eggs of each individual (as well as the means of the
different individuals) vary less in respect to the dimensional
characters than in respect to weight. The eggs must vary
simultaneously in both dimensions.

Whereas a comparison of the means for the different individ-
uals shows that intra-racially the eggs are more uniform in
yolk weight than in the weight of the other parts or of the
whole egg, within the eggs of the individual the yolk weight is
almost as variable as the shell weight and much more variable
than the weight of the albumen or of the whole egg.

THE RELATION OE EGG CHARACTERS TO OTHER CHARACTERS.

The factors which bring about the individuality in the eggs
of the different birds are too complex for analysis from the
data in hand. Other facts known about the birds were investi-
gated to discover possible relationships between the variation
in egg characters and variation in other characters. The re-
sults were as follows :

1. The early hatched birds laid earlier in the fall than those
hatched during the middle of the season, while the late hatched
birds did not lay until spring.

IIO MAINE AGRICULTURAL EXPERIMENT STATION. I9H-

§

2. Among the birds which began to lay in the fall there is
no relation between the number of eggs laid and the time of
hatching or the exact date of the first egg.

3. Among the whole flock there is no relation between the
mean size of the egg and either the time of hatching or the
time of laying the first egg.

4. Correlations calculated between body weight and number
of eggs and size of eggs, and between number of eggs and size
of eggs were all significantly zero.

Autopsy data were taken on the thirteen birds which com-
pleted their second year. The number of birds is too small for
statistical analysis. They show an apparent relationship be-
tween the size of the inactive oviduct and the time which has
elapsed since the last egg and the amount of yolk present in the
ovary. The size of the active oviduct appears to be related to
the body size. It is not possible to determine whether or not
there is a relationship between the size of the active oviduct and
the size of the egg.

The: Interrelation oe the Dimensions, the Shape, Size of
Egg and the Size and Proportion of the Parts.

The individuality of the birds in respect both to the mean .
size and to the amount of variation of each egg character was
discussed in preceding paragraphs. In this discussion it was
noted that there was a general relationship between the various
characters, that is when an individual laid large eggs her char-
acterisitc egg was both long and broad and each one of the
component parts was relatively large. It was also noted that
the eggs of an individual evidently varied coincidently in both
diametral dimensions since they were less variable in either
dimension than in size. It was further noted that within an
individual there was a compensatory variation in the size of
the parts since the variation in the whole egg is less than the
variation in the parts.

The purpose of the present part of the paper is to show the
interrelation or more strictly correlation of the various egg
characters within the eggs of each of the several individuals.

The results of an analysis of the data may be summarized as
follows :

SIZE, SHAPE AND PHYSICAL CONSTITUTION OF THE EGG. Ill

1. Different pairs of egg characters show a decided differ-
ence in the degree of correlation.

2. There is a general tendency for a given pair of charac-
ters to be similarity related in the eggs of the several individ-
uals.

3. Individuals, however, show significantly different degrees
of correlation in any pair of characters. The range of the
individual variation is wider and the number of individuals
which differ significantly from each other is larger for some
pairs of characters than for others.

4. Length and breadth are positively but not highly cor-
related.

5. Both length and breadth are highly positively correlated
with weight.

6. Breadth-weight correlation is as a rule higher than length-
weight correlation but an individual may show the reverse
relation.

7. Index and weight correlation when significant is negative
and low.

8. The relation between length and breadth, length and
weight, and index and weight is much closer within the eggs
of an individual than within the eggs of the strain, but breadth
and weight are as closely related within the eggs of the strain
as within the eggs of an individual.

9. In eggs of the same weight the two dimensions are nega-
tively correlated but the degree of correlation varies greatly
in the different individuals.

10. Either dimension is highly correlated with weight when
the other dimension is constant. The correlation between
breadth and weight is higher than the correlation between
length and weight. The individual variations are less than in
the gross correlations or in the correlations between length and
breadth in eggs of the same weight.

11. The weight of each of the three parts of the egg (albu-
men, yolk and shell) is positively correlated with egg weight.
The heaviest part, albumen, is invariably very closely correlated.
Yolk is also closely correlated but there is a greater individual
variation in the value of the coefficients. Shell, the lightest part,
is much less closely correlated and the individual differences
are much greater than in the case of the two heavier parts.

1 12 MAINE AGRICULTURAL EXPERIMENT STATION. I914.

12. The correlation between either of the two large parts
and the whole egg when the other part remains constant is high
and shows insignificant individual differences. The albumen-
weight correlation is slightly* higher than the yolk-weight
correlation.

13. The correlation between albumen and yolk is higher than
the correlation between either of these and shell. In general
the albumen shell correlation is higher than that for yolk and
shell. The range of correlation for any of these pairs of char-
acters, however, varies in different birds from insignificance to
high correlation.

14. In eggs of the same weight any two parts are negatively
correlated. The yolk-albumen correlation is highly negative
and the different individuals show little variation in the value
of the coefficient. The yolk-shell and albumen-shell correla-
tions are significantly negative for most individuals but the
value of the coefficients for the different individuals varies
greatly.

15. Length and breadth are positively correlated with the
weight of each part of the egg. When there is a significant
difference between the correlation of length and any part ana
the correlation of breadth and the same part the breadth corre-
lation is higher.! There is, however, a great deal of individual
variation in the degree of these relationships.

16. Albumen and yolk correlations with either dimension
are higher than the shell correlation. Albumen correlations are
significantly higher than yolk correlations for several individ-
uals. The reverse is true in only one case.

17. In eggs of the same weight the correlation of either
dimension with any part is often not significant. The different
birds show a great deal of variation in these coefficients. The
most usual relationship is for breadth to be positively correlated
with yolk and negatively correlated with albumen and shell.

An Analysis oe the Variation Among the Eggs oe the

Same Bird.

It has been shown that while the eggs of an individual
resemble each other more closely than do eggs forming a

* This difference may not be significant,
t There are two exceptions to this statement.

SIZE, SHAPE AND PHYSICAL CONSTITUTION OF THE EGG. 113

random sample from other birds of the same strain, there is
nevertheless a high degree of variation among them. This
variation is appreciable in every egg character but the several
characters show it in decidedly different degrees. The means
of the individual coefficients of variation are given in Table I.

TABLE 1.

Showing the Mean of the Individual Coefficients of Variation

for each Egg Character.

Egg Character.

Mean of the Individual Coefficients
of Variation.

Shell weight

10.43

Yolk weight

9.84

Albumen weight

6.43

Egg weight

6.36

Length

3.32

Breadth

2.38

The purpose of the present part of the paper is to investigate
further this variation among the eggs of the individual.

The data collected in the present investigation offer material
for the study of the following phases of this variation in the
eggs of the individual. First the variation due to the age or
maturity of the bird, second that due to the seasonal distribu-
tion of the laying, third that due to the general physiological
conditions, i. e., health of the bird, fourth variation due to the
tendency of the birds to lay in litters and fifth that related to
the tendency to lay in smaller groups or clutches.

I. THE VARIATION RELATED TO THE AGE AND MATURITY OF THE

BIRD.

The data used in this study are the measurements taken on
all the eggs laid by each of the twelve birds which lived and
continued to lay through two years. To illustrate certain points
it is possible to supplement this with the data on the eggs of
other birds which did not complete their second year.

The progressive change in egg weight associated with the
maturity of the bird is shown by the monthly mean egg weight.
In the case of each individual the egg weight at the end of the
second year is decidedly greater than at the beginning of the
first year. From the beginning of laying until the beginning of
the first breeding season the eggs increase rapidly in size. From
this time on the weight of the egg is subject to quite perceptible

2 89026

1 14 MAINE AGRICULTURAL EXPERIMENT STATION. I9I4.

seasonal fluctuations which will be discussed later ; but with
very few exceptions the eggs of the second year are decidedly
larger than the eggs of the corresponding months of the first
year. That is the eggs of a bird continue to increase in weight
with the increased maturity of the bird at least up to the end
of the second laying year. After the beginning of the first
breeding season the increase in weight is much slower than dur-
ing the few months which precede.

The weight of the egg is made up of the weight of the three
parts albumen, yolk and shell. Each of these parts are also
variable in weight. In fact, each is more variable than the
weight of the whole egg.

A study of the progressive change in each of these parts can
be made in a manner similar to the above study of egg weight.

The monthly mean yolk weights show ever more uniformily
than, the weight of the whole egg that the weight increases
rapidly to the beginning of the first breeding season and from
that time on more slowly. The yolk weight shows the seasonal
fluctuations shown by the egg weight; but their magnitude is
smaller.

The monthly mean albumen weights show a less abrupt rise
during the first three months and greater seasonal fluctuations
than the monthly mean egg weights or yolk weights. In gen-
eral, however, they show an increase in albumen weight with an
increase in the age of the bird.

The monthly mean shell weights are larger for the second
than for the first year but they show a large proportional varia-
tion which is usually related to the variation shown by the
other parts but may be independent and in the opposite direc-
tion. For example during the first three months laying all but
one of the birds show a decrease in shell weight which is as
decided as the increase in the weight of the other two parts.

In order to generalize the study of the increase in the weight
of eggs and of each of its parts as the bird grows older it is
desirable to bring together the results of the study on the
individual birds.

The birds used in this investigation form a more homoge-
neous group than is often available for statistical work. They
were “pure-bred” in the fancier’s sense and line-bred and were
within a few weeks of the same age. They had lived under the

SIZE, SHAPE AND PHYSICAL CONSTITUTION OF THE EGG. 115

same environmental conditions since hatching and they began to
lay at very nearly the same time. Further there are only
slight differences in the degree and direction of variations in
the monthly means of the same weight character for the sev-
eral birds.

The material has been brought together by calculating di-
rectly for each weight character the monthly mean of all the
eggs laid by the twelve birds.

The means are given in Table 2.

TABLE 2.

Showing for Bach Month the Number of Birds Laying, the
Number of Bggs Laid and the Mean Egg Weight, Yolk
Weight, Albumen Weight and Shell Weight for the Twelve
Birds which Completed their Second Year.

Month.

No. birds
laying.

No. eggs
laid.

Mean

egg

weight,

gms.

Mean

yolk

weight,

gms.

Mean

albumen

weight,

gms.

Mean

shell

weight,

gms.

Mean tem-
perature
in degrees,
F.

1910

October

1

3

48.56

13.66

28.13

6.17

48.40

November

12

137

47.48

12.87

29.11

5.46

34.95

December

11

206

48.99

13.39

30.38

5.22

19.55

1911

January

12

110

50.37

14.24

30.88

4.84

18.15

February

10

100

53.53

16.27

30.94

5.37

14.56

March

12

224

54.25

16.18

32.46

5.63

25.20

April

12

210

54.42

16.43

32.48

5.54

40.90

May

12

192

53.73

16.23

32.12

5.39

59.25

June

12

144

53.66

16.25

32.23

5.13

62.30

July

11

158

52.22

16.31

30.83

4.87

71.20

August

11

170

54.75

16.72

32.53

5.46

66.20

September

11

132

55.44

16.88

33.13

5.41

57.20

October

9

86

56.68

17.61

33.23

5.55

46.10

November

4

15

58.51

17.41

35.57

5.43

34.40

December

4

43

59.20

17.50

35.93

5.77

28.00

1912

January

5

29

61.30

17.90

37.72

5.67

10.90

February

8

39

54.73

17.85

31.88

5.08

17.70

March

11

179

59.03

18.46

34.71

5.68

27.00

April

12

130

56.12

17.47

33.10

5.08

43.50

May

12

138

55.63

17.71

33.47

5.31

57.10

June

11

148

58.10

18.45

34.30

5.62

63.80

July

12

137

56.97

18.31

33.46

5.32

67.20

August

10

88

57.16

18.16

33.61

5.39

62.50

September

11

123

58.46

18.19

34.69

5.53

57.50

October

9

83

61.64

18.19

37.20

6.23

49.80

Il6 MAINE AGRICULTURAL EXPERIMENT STATION. I9I4.

From this table it is seen that in the case of the flock, as
in the case of individual birds, the yolk weight increases more
constantly than the weight of the other two parts or of the
whole egg. That is it shows smaller irregular fluctuations.

The flock monthly mean yolk weight increases very rapidly
at first. The increase in weight is actually greater during the
first four months than during the next twenty months. The
yolk weight, however, continues to increase during the second
year. This suggests that the increase in yolk weight is follow-
ing a logarithmic curve of the form y=A-\-Bx-\-C log x where
yz^yolk weight, x— time, and A, B and C are constants. This
type of curve was fitted as a first trial.

The resulting curve is

y— 12.6728 — .0261^+4.5669 log x.

When the ordinates of this curve are calculated and plotted
together with the observation curve the smooth curve shows an
excellence of fit which indicates that the increase in weight of
the egg yolk in these birds is in fact expressed analytically by
this logarithmic curve. In other words this shows that the
mean yolk weight increases with each successive month from
the beginning of laying at least to the end of the second year
but the rate of increase diminishes with the successive months.

The data on which these calculations were based were taken
from the eggs of birds which began to lay at an age of from
five and one-half to six and three-fourths months. It is inter-
esting to know the size of the egg yolks at the beginning of
laying and the direction and rate of change in yolk weight when
a bird is older or younger than this at the time she begins to
lay. For this reason three birds that began to lay February
23 were added to the individuals under investigation. These
birds were eight and three-fourths to nine months old, i. e., they
were from two to three and one-half months older than those
pullets just discussed. It is not so easy to investigate pullets
which begin to lay younger, as the Barred Plymouth Rocks, at
least under the methods of hatching and handling employed at
this station, rarely lay before they are five months old. We
are indebted to Mr. Walter Gerald of Unity, Maine, for the
opportunity to obtain data on the first ten eggs of a pure bred
Barred Plymouth Rock pullet which began to lay when she was
exactly three months old and before she had moulted her

SIZE, SHAPE AND PHYSICAL CONSTITUTION OF THE EGG. II/

chicken feathers. The eggs of Mr. Gerald’s pullet were very
small and the yolks were only about one-half the size of the
yolks of the five to six months old pullets. Also the yolk weight
increased considerably during the fifteen days observations.

The case of this precocious pullet adds evidence to the view
suggested by the data from the flock investigated, namely thar
the egg yolks increase in weight as the bird1 matures and that
this increase is more rapid at early than at later stages of de-
velopment. There is further evidence of this fact from the
data on the birds which began to lay in February. These birds
were from eight and three- fourths to nine months old when
they began- to lay.

The relation of the age of the bird at the beginning of laying
to the size of the first yolk is shown by the data given in table 3.

TABLE 3.

Showing the Relation of the Age of the Bird to the Size of

the 1st Yolk.

Bird No.

Age at laying
1st egg.

Weight of 1st
yolk.

Mean.

Mr. Gerald’s pullet,

3

6.21

6.21 (1 bird).

218

5§

10.19

516

5f

10.75

10 . 945 (4 birds).

478

51

9.30

236

5|

13.74

446

6

13.09

184

6

12.22

441

6

12.63

259

61

10.68

212

61

11.74

235

61

14.05

192

61

10.51

204

61

12.27

12.236 (13 birds).

198

61

11.53

139

61

11.51

211

61

12.28

205

6f

13.22

243

61

13.35

137

7

13.75

172

71

13.27

13 . 51 (2 birds).

459

81

14.19

489

81

17.63

514

(

9

15.11

15 . 643 (3 birds).

Data on the eggs of the birds which began to lay in February
show also that in these cases the yolk weight does not begin
small and increase very rapidly, as in the case of the bird?
which begin to lay in the fall. In fact with one exception the
yolk weight shows a slight decrease for the first few months.

Il8 MAINE AGRICULTURAL EXPERIMENT STATION. I914.

This is evidently a seasonal decrease as it is shown by several of
the birds that began to lay in November. Unfortunately none
of these birds laid through the second year but up to within a
month of death these birds showed monthly means very similar
10 the means for others of the flock during the same months.

Another bird (No. 236) laid one egg in November but did
not lay again until March 31. She may therefore be considered
as a bird without laying experience until she was practically
nine and three-fourths months old. The one egg in November
however indicates the size of eggs she would have laid at that
period.

A comparison of the monthly mean yolk weight for these
four birds with those for the other birds of the flock shows that
the first eggs laid by birds which do not lay until they are nine
months old have yolks as large as the yolks of the eggs laid at
the same time by birds of practically the same age which began

to lay some months earlier. The rate of change in yolk weight
also is similar to that shown during the same time by the birds
which had laid younger. The one observation in November
for bird No. 236 is also in accord with the observations for the
other birds at that time.

It is also interesting to note that the yolk weights shown by
any bird at any month are dependent upon the size typical for
the individual and upon the time of year. It is apparently inde-
pendent of whether the bird has been laying during the previous
months, or, in fact, of whether or not she has ever laid before.
That is the increase in yolk weight does not seem to be due to
a perfection of the morphe genetic activity due to physiological
practice but seems rather to be due to the stage of development
or differentiation of the individual.

Since the birds used in this investigation were very nearly
the same age it is not possible to separate absolutely the effects
of the age of the bird and of the season of the year. Certain
undoubted seasonal variations will be discussed later. It seems
however that the general tendency for a continued increase in
yolk weight at a constantly diminishing rate through all sea-
sonal conditions must be due to the general stage of maturity
of the bird. The small but rapidly increasing yolks in the pre-
cocious pullet also supports this view. This gradually diminish-
ing increase in yolk weight is represented by a logarithmic

SIZE, SHAPE AND PHYSICAL CONSTITUTION OF THE EGG. 119

curve. This curve which represents analytically the law of the
increase in yolk weight with the age of the bird is of the same
type as the curves which have been found to fit various kinds
of growth data in both animals and plants.

The first interpretation that suggests itself is that this in-
crease in the size of the yolk is a direct effect of the analogous
increase in the size of the individual. That the body weight as
well as yolk weight normally continues to increase at least to
the end of the second year was seen by comparing the body
weights at the beginning, middle and end of the observations.
However, the curve showing the increase in yolk weight is not
parallel to a curve which shows the increase in body weight at
the same period. During the period of most rapid increase in
yolk weight the increase in body weight was too small to be
certainly distinguished from the fluctuations due to temporary
variations in the amount of food and waste present in the
body. This indicates that the stooge of development or differ-
entiation which determines the size of yolk is not accurately
measured by the body weight . It is of course well known that
the different organ systems of the body show different growth
stages at the same time and that the reproductive system shows
most rapid growth not long before the beginning of its func-
tional activity.

In yolk weight we find a logarithmic approach of successively
formed structures to a type. Pearl* (’07) found this true of
leaf number per whorl in Ceratophyllum and later (’09) t of
egg shape in a domestic fowl. In the case of yolk weight ,
however, it is certain that this is not due to the continued pro-
duction of the like parts but to the condition of the individual
at the time the part is produced.

The change in yolk weight due to the age or maturity of
the bird has been discussed at length because the weight of this
part of the egg seems to be most closely related to the age of
the bird and least affected by the other natural causes of vari-
ation in egg size. The weight of albumen and shell also both

* Pearl, R., Pepper, O. M. and Hagle, F. H. Variation and Differen-
tiation in Ceratophyllum, Carnegie Institution Publ. No. 58, pp. 1-136.
1907.

t Pearl, R. Regulations in the Morphogenetic Activity of the Ovi-
duct. Jour. 'Exp. Zool., Vol. VI, 1909, pp. 339-358.

120 MAINE AGRICULTURAL EXPERIMENT STATION. I914.

increase with the age of the bird but the fluctuations a**e much
greater than in the case of yolk weight. Since the weight 0£
each of the parts increases with the age of the bird the weight of
the egg must show this relation also. In the case of the yolk
weight we have seen that the rate of increase is quite precisely
logarithmic. The fluctuations from this type are small. Albu-
men weight shows a tendency to this type of increase but cer-

A

tain fluctuations are very great. The increase in shell weight
is not logarithmic. The weight of the whole egg shows an
increase in weight distinctly logarithmic although less precisely
so than yolk weight.

The different nature of the physiological processes involved
in the formation of the parts of the egg affords ample basis
for an independent variation. The weight of the yolk is deter-
mined by the amount of yolk deposited while it is within the
ovarian follicle. The albumen weight depends upon the amount
of secretion of the albumen glands of the entire oviduct, while
the weight of shell depends only upon the secretions of the
shell forming glands of the uterus. It is conceivable that a
disturbance of the processes involved in the formation of one
part of the egg may have little or no effect on the processes
involved in the formation of the other parts. The results given
above indicate that the factors which determine the size of
the yolk are less disturbed by temporary conditions than are
those which determine the amount of albumen and shell secre-
tion.

2. VARIATION RELATED TO THE SEASON OE THE YEAR.

Yolk weight and albumen weight show in general similar sea-
sonal variations. Both parts increase in weight from the begin-
ning of the fall laying to the beginning of the breeding season
in February or March. This increase is proportionately greater
in albumen weight than in yolk weight. At the beginning of the
breeding season the yolk weight increases while the albumen
weight either remains constant or decreases. The yolk weight
remains nearly constant during the breeding season and the
following summer, but the albumen weight tends to fall off
especially as the molt is approached. Thus the proportion of
yolk in the eggs during the spring and summer is larger than
during the fall and early winter.

SIZE, SHAPE AND PHYSICAL CONSTITUTION OF THE EGG. 1 21

When the monthly mean shell weight is compared month by
month with the monthly mean albumen weight many of the
same fluctuations will be noted. There is one decided exception.
During the first three months shell weight is decreasing propor-
tionately more rapidly than the albumen weight is increasing.
The fluctuations shown at other seasons are proportionately
much greater in shell weight than in albumen weight.

The variation in weight which is associated with the change
in season is to a certain extent similar in the case of each part
of the egg. The parts, however, vary greatly in the propor-
tional magnitude of the seasonal variations. The difference is
so great that the proportion of the parts is altered in different
seasons. Yolk weight is apparently least affected by these sea-
sonal factors. Albumen weight shows very large fluctuations
which have a decided tendency to repeat themselves in succes-
sive years and therefore appear to be undoubted seasonal fluc-
tuations. The fluctuations in shell zoeight are proportionately
greater than in either of the other parts , but do not show the
same tendency to repeat themselves in the successive years. It
therefore seems possible that some of the large fluctuations in
shell weight may be due to some other circumstances than the
normal change in season.

Showing that certain changes in yolk weight and albumen
weight are correlated with change in season does not indicate
the fundamental cause of these changes. The solution of this
problem must be by controlled experiments. Two> interpreta.-
tions suggest themselves. I. The fluctuations may be due
to the direct effect of seasonal changes in environment. 2.
They may be dtie to the general changes in metabolic processes
which also find expression in the differentiation of a breeding
season and a fall molt, and which are probably due in part to
environment and in part to heredity.

In general it seems probable that while the seasonal fluctua-
tions in egg weight may be to some extent the direct effect of
environmental conditions, yet to. a much greater extent they are
the indirect effect of such conditions. That is they are due to
general physiological changes in the individual.

122 MAINE AGRICULTURAL EXPERIMENT STATION. I914.

3. VARIATION RELATED TO THE STATE OE HEALTH.

One of the birds (No. 441) used in this investigation offers
an opportunity to study the effect of the state of health upon the
weight of the egg. This bird laid 179 eggs during her pullet
year. These eggs increased in size exactly in accordance with
the general rule for the flock. Her body weight increased1 from
2003 gms. in November and December 1910 to 2325 gms. in
September, 1911. That is up to the first fall month she was a
normal bird. She stopped laying for the molt October 17, 1911,
and did not lay again until March 4, 1912. From this time
until August 17 she laid quite well for a second year Barred
Plymouth Rock, producing 90 eggs in this time. These eggs
were smaller than the eggs of the previous year. Nothing ab-
normal was seen in the appearance of the bird until during the
summer when she began to appear somewhat sick. She con-
tinued to lay until August 17. After this she became more
dumpish. In September it was apparent that she would not
recover and lay before the close of the investigation in Novem-
ber. She was therefore, killed and autopsied September 10,
1912.

At autopsy her body weight was 2190 gms., a slight decrease
from the weight a year before. The following lesions were
recorded: 1. The liver was congested and friable. 2. The
lungs were congested and showed yellowish lesions. 3. There
was some peritoneal disturbance which had caused a greenish
deposit on the intestinal mesentery. That is the bird at autopsy
showed evidences of disturbances which were probably of some
time standing.

A comparison of the egg data for this bird with the cor-
responding data for the other birds show that during the first
year each egg part increased in weight in the normal manner.
The bird was then no doubt in good physical condition. The
bird did not lay from October, 1911 to March, 1912. Through-
out the second year the weight of each egg part decreased. This
decrease was most rapid after the bird was observed to be sick
but had been going on for three months before this. Evidently
the physiological disturbances had affected the size of the egg
before they affected the behavior of the bird.

Five other birds died or were killed during the investigation.

SIZE, SHAPE AND PHYSICAL CONSTITUTION OF THE EGG. 123

None of these were affected 'by long standing diseases and none
showed a diminution in egg size greater than normal variation.

It is clear that a diseased condition of a bird may cause a
decrease in the weight of each part of the egg without causing
a cessation of laying. It is also clear that other disturbances
sufficient to cause death may not affect the size of the egg pro-
duced on the day of death.

Fere* studied the effect of morphine intoxication on the
weight of the egg. He found that a stupefying dose caused a
diminution in the weight of the egg which was laid on the
clay of the intoxication and which must have' been nearly
formed at the time of the injection of the morphine.

The effect of the morphine interrupted the laying for four
days and when it was resumed the first two or three eggs were
smaller than before the intoxication. This shows that an ex-
perimentally altered physiological condition may cause a dimi-
nution in the weight of the egg.

It has been shown that, in general, the monthly mean egg
weight increases with the age of the bird, fluctuates with the
season of the year and is affected by the state of health. The
fact now to be considered is that during any month the eggs ot
each bird show a very considerable variation in egg weight and
in the weight of each of the egg parts.

Before considering the nature of this variation in the weight
of the successive eggs it is important to notice the following
points in regard to the rate of production of the bird.

1. Periods of production alternate with periods of non-
production. That is the bird lays in litters.

2. The length of the periods of production and the number
of eggs laid in one litter vary at different seasons of the year.

3. Throughout the warm months (April to September) the
period of non-production is typically a period of broodiness.
That is the instinct of the bird is to lay a litter of eggs and
then incubate them. While thirteen of the birds show this in-
stinct, nine were never broody. It is also interesting that
broodiness or non-broodiness is typical of the individual, for

* Fere, M. Ch. Note sur la puissance toxiane et la puissance tera-
togene de la morphine sur le poulet. Bull, et mem. de la Soc. med. des
hop. de Paris, Vol. 14, series 3, pp. 608*617, 1897.

124 MAINE AGRICULTURAL EXPERIMENT STATION. I914.

when a bird was broody the first year she was also broody the
second year and if she was not broody the first year she did
not show this instinct the second year. Typically a bird which
was broody at all was broody four or five times during a season
but the number of times varied from I to 7 with the indi-
vidual. The nine birds which were never broody show all de-
grees of a tendency to lay in litters from zero in the extreme
case of one bird which laid from February to the end of the
year at a nearly uniform rate, i. e., in clutches of 1 to 3 usually
two eggs separated by one or two days on which no egg was
laid, to the opposite extreme of a bird which had exactly the
same definite periods of non-production as the birds with typi-
cal broody periods but she lacked the instinct to incubate the
eggs.

4. Within a litter the laying is broken up into clutches. The
eggs of a clutch are laid on successive days. The clutches ire
separated by one or two (occasionally more) days on which no
eggs are produced.

5. The number of eggs in a clutch and the number of days
between clutches vary in the different birds in the same season
and in the same bird at different seasons. During the part
of the year when the bird is not laying in litters succeeded by
broody periods the size of the clutches is small. The birds
which lay continuously (i. e., do not lay in litters) lay in small
clutches throughout the season. During the part of the year
when periods of production alternate with periods of broodiness
the litters show a decided tendency to start and end with small
clutches while the number of eggs in the intermediate clutches
is larger. An individual may nevertheless start or end a litter
with large clutches.

These facts, in general, accord with the hypothesis of Pearl
and Surface.* “The actual visible egg production in each in-
dividual bird tends to occur in definite cycles or periods of
varying lengths, alternating with non-productive periods.”

“The rate of fecundity (amount of egg production per unit
of time conceived in the sense of the differential calculus) is
in any bird a minimum at the beginning of the cycle of produc-

* Pearl, R. and Surface, F. M. A Biometrical Study of Egg Pro-
duction in the Domestic Fowl. II. Seasonal Distribution of Egg Pro-
duction. U. S. Dept, of Agr., Bui. An. Ind. Bui. no, Pt. II, 1911.

SIZE, SHAPE AND PHYSICAL CONSTITUTION OF THE EGG. 12$

lion, increases to a maximum at what may be termed the height
of the cycle, and decreases to a minimum as the end of the
cycle is approached.”

In order to make a study of the variation in the physical
characters of the successive eggs of an individual in relation to
the variation in her rate of production, the weight of each egg
and of each of its parts was plotted as ordinate and the date
of laying as abscissa. Such diagrams were made for each bird.
A study of these shows the relation of the weight of the egg
and of each egg part to its position within the clutch and
litter.

4. REFLATION OF THF WEIGHT OF THE FGG TO ITS POSITION IN

IN LITTFR.

Fere’s t statement that, in general, the eggs at either end of
the litter are smaller than the intermediate ones but that this
relation is not absolute is verified by the observations on the
birds used in the present investigation. In fact the weight of
the egg in relation to the position within the litter approximates
a curve of the type of the hypothetical “rate of fecundity”
curve proposed by Pearl and Surface * (1911). It thus seems
possible that the same conditions which cause fluctuations in
the rate of production may also be responsible for the cyclic
fluctuation in the egg weight. It is certain, however, that the
weight of the egg is also related to the position of the egg in
the clutch and that the position of the egg in the clutch and in
the litter may have opposite effects on the size.

5. REFLATION OF WEIGHT OF THF FGG TO ITS POSITION IN THF

CLUTCH.

When an egg was produced on each day for a number of
successive days, as a rule, the eggs decreased in weight from
the first egg to the end of the series. The first egg after a day
on which no egg was laid was larger than the egg at the end of
the preceding series. That is the eggs of the clutch decrease

t Fere, M. Ch. Note sur le poids Foeuf de poule et sur variation
dans les pontes successives. Journal de l’Anatomie et de la Physiologie.
T. 34, pp. 123-127, 1898.

$ Pearl and Surface. Loc. cit.

\

I

126 MAINE AGRICULTURAL EXPERIMENT STATION. I914.

in size from the first egg. The first egg of a new clutch is
larger than the last egg of the preceding clutch. There are a
few exceptions to this especially where a litter begins with a
long clutch.

The fact that in general the eggs laid on successive days de-
crease in size while after a day or two on which no egg is
produced the egg size increases seems to indicate that there
is either an exhaustion of material available to elaborate into
the various egg parts or that there is a fatigue of the reproduc-
tive organs which causes a decrease in the amount of material
elaborated. A variation in the supply of available materials
or a variation in the physiological tone of the organ would ac-
count for the cases of unusual size relations between succes-
sive eggs.

It has now been shown that the variation in egg weight is
related to the age of the bird, the season of the year, the state
of health and the rate of egg production.

The weight of all the eggs and egg parts for the individual
birds for two years furnish material to test Pearl’s* (’07)
“second law of growth” i. e., that the variability of succes-
sively produced like parts decreases with the number of such
parts produced. If this law was operating during the produc-
tion of successive eggs the magnitude of the fluctuations in egg
weight would decrease with the number of eggs laid. The dia-
grams described on page 125 show that the fluctuations in
egg weight and weight of each of the egg parts are as large
at the end of the second as at the beginning of the first year.
It is evident then that this law does not hold for the variation
in weight of successive eggs. Pearl t (’09) found no evidence
that this law was acting in regard to the shape of the eggs of
a fowl on which he studied regulation in shape.

This section of the paper has shown that the weight of any
part of a bird’s egg depends, first upon the hereditary constitu-
tion of the bird, second her physical constitution or state of
health, third, her stage of development, fourth, the season of
the year, and fifth, the position of the egg in its clutch and

* Pearl, R. Loc. cit.
f Pearl, R. Loc. cit.

SIZE, SHAPE AND PHYSICAL CONSTITUTION OF THE EGG. I2JT

Utter. It has also shown that the variation in successive eggs
is as great at the end of the second as at the beginning of the
first year’s laying.

Discussion of Results.

Only a beginning of the analysis of the factors which produce
variation in the eggs of the domestic fowl is possible from
the results of this investigation.

Since the individual variation in egg characters is less than
race variation, and since the association of various pairs of
egg characters is greater within the egg of an individual than
within the eggs of the race, it is concluded that an individual
inherits or, at least, possesses at sexual maturity, the tendency
to lay eggs of a certain particular and individual shape, size
and physical constitution.

The seasonal and cyclic fluctuations in the eggs of the indi-
vidual on the other hand show that this predisposition toward
eggs of a particular type and size may be influenced by physio-
logical and possibly also environmental conditions.

That the individuality in egg characters is related to individ-
uality in other characters seems certain. The fact that rhe
large varieties of hens lay larger eggs than bantams indicates
that within broad limits the size of the egg is related to body
size. The insignificance of the correlation coefficient between
egg size and body size for the egg of the individuals studied
however shows that this relation does not necessarily obtain
within narrow limits. It is not possible to decide the limits of
the relation of body size to egg size from so small a number
of individuals. The point is open to further investigation.

In the study of egg characters it must be kept in mind that
eggs are quite different material than is usually employed in
biological studies of variation. They are not organs or parts
of ^organs that owe their size and shape to growth, i. e., the
proliferation of the cells of which they are made up. Each
egg is one modified cell and represents the accumulated results
of the activity of a great many cells, not a part of it. The
different parts of the egg arise from different organs or parts
of organs (the ovary and the parts of the oviduct) and by
quite distinct physiological processes (yolk deposition and
albumen, membrane, and shell secretion.)

128 MAINE AGRICULTURAL EXPERIMENT STATION. I914.

The egg then represents not a normal organic part of the
individual but a discrete unit of production of certain corre-
lated organs. The size and form of such units are to a cer-
tain extent dependent upon the size and form of these organs.
But the size, form and frequency are necessarily also dependent
upon the physiological tone of the organs and of the entire
organism.

It is not strange that the same individual at different
seasons and different individuals throughout the year may
show variations in egg size and in the number of eggs produced
which are out of proportion to the variation in body size. It
is quite possible that egg production like milk production * is
related to the amount of food consumed above maintenance.
That is the bird which lays large eggs and many of them is
one which in addition to the organic potentiality to lay eggs
of this size possesses also both the physiological capabilities of
digesting a large amount of food above the amount required
for the maintenance of the body and of using this absorbed
food for the production of yolk, albumen, etc. The fact that
the same individual lays larger eggs and more of them at cer-
tain seasons than at others may be due to the fact that she is
capable of digesting and utilizing for egg production more
food at those seasons. It has in fact been shown by Rice **
that the number of eggs produced by a flock is positively cor-
related with the amount of food consumed, and the fluctuations
which he found characteristic for the amount of food consumed
are very similar to the seasonal fluctuation in egg weight. The
work of Riddle t on yolk formation also supports this view.

In general the conditions which favor the production of a
large number of eggs also favor the production of large eggs
(i. e., large for the particular individual). Yet there are

* Eckles, C. H., and Reed, O. E. A Study of the Cause of Wide
Variation in Milk Production in Dairy Cows. Mo. Agr. Expt. Sta. ’Re-
search Bui. No. 2, 1910, ipp. 107-147.

** Rice, J. E. The Moulting of Fowls. Cornell Univ. Exp. Sta. Bui.
258, 1908, pp. 211-68.

f Riddle, Oscar. On the Formation Significance and Chemistry of
White and Yellow Yolk of Ova. Journal of Morphology, Vol. 22, 1911,

pp. 456-484.

SIZE, SHAPE AND PHYSICAL CONSTITUTION OF THE EGG. 1 29

limits to this relationship. When a number of eggs are pro-
duced in succession each successive egg is smaller than its pre-
decessor. There is probably either an overdemand on the
amount of available material or there is a fatigue of the repro-
ductive apparatus which hinders the formation of the egg parts.

A study of the interrelation of the different parts of the egg
is a study of the correlation of the activities of the different
organs and parts of organs by which they are formed. The
fact that the oviduct of a bird approaching a period of laying
enlarges as the yolks enlarge has been long recognized. The
observations on the domestic fowl at this laboratory agree in
general with those of Bartelmez * on pigeons. There are
nevertheless some specific differences between fowls and
pigeons. In a bird in the non-laying condition the oviduct is
a small straight tube and there are no oocytes in the ovary
which contain a perceptible amount of yolk. About the time
a group of oocytes enters upon the final growth period (when
they begin to enlarge above 6 mm.) the oviduct begins to en-
large. The stimulus which initiates these activities in fowls is
not known. There is no evidence that it is connected with
mating as has been shown for the pigeon.! The presence of the
male is certainly not necessary and no behavior on the part of
a fowl has been observed to indicate that she is physically
mated with another female or with anything else. Whatever
the stimulus that starts the reproductive mechanism going, it
is true that while the first yolk is forming the oviduct enlarges
to functional size.

This correlation between the ovary and oviduct is now com-
monly attributed to the action of the internal secretion of the
ovary. Bartelmez * states that “interstitial cells of the ovary
show much greater signs of activity in functioning ovaries
than do those in ovaries of birds that had not laid for a long
time.” That the enlargement of the oviduct is due to the in-

* Bartelmez, George W. The Bilaterality of the Pigeon’s Egg. Jour,
of Morph., Vol. 23, 1912, pp. 270-310.

t Harper, E. H. The Fertilization and Early Development of the
Pigeon’s Egg. Am Jour. Anat., Vol. 3, 1904, pp. 344-381.

Craig, W. Oviposition Induced by the Male in Pigeons. Jour, of
Morph., Vol. 22, 1911, pp. 299-305.

t Bartelmez, G. W. Loc. cit.

130 MAINE AGRICULTURAL EXPERIMENT STATION. I914.

ternal secretion of the ovary has not been completely demon-
strated by any means. Certain observations made in connec-
tion with other researches make the conclusion somewhat doubt-
ful. They certainly show that enlargement of the oviduct is
not necessarily connected with yolk formation although this is
the normal relation.

The observations referred to are :

1. The oviduct of the hermaphrodite fowl described by
Pearl and Curtis * was at autopsy in essentially the same con-
dition as that of a normal fowl that had recently completed or
was soon to begin an egg laying period. Yet this bird had never
laid and the histological examination of the ovary failed to
demonstrate the presence of oocytes in any stage of develop-
ment. The ovary of this bird was about the size of a function-
ing ovary after the large yolks have been removed. It was com-
posed of a highly cellular stroma like tissue penetrated from
the stalk by a very vascular connective tissue and covered ex-
ternally by a layer of peritoneum. The presence of interstitial
cells could not be demonstrated, but this may have been due
to poor fixation.!

2. In the routine autopsy work in this laboratory there has
been noted several cases of birds which had not laid for a
long time and which had no oocytes with any yolk but which
possessed nearly functional sized oviducts. There were in
these cases ovarian tumors which seemed here to be associated
with the enlarged oviducts. None of these ovaries were exam-
ined for interstitial cells which may or may not have been
present.

That the mere increase in weight of the ovary may stimu-
late the enlargement of the oviduct is not altogether impossible.
These pathological ovaries may however have furnished the
normal internal secretion in quantities sufficient to cause the
enlargement of the oviduct. It is nevertheless certain 'that
whatever the stimulus may be it is not absolutely dependent

* Pearl, E., and Curtis, M. R. Studies in the Physiology of Reproduc-
tion of the Domestic Fowl. A 'Case of Incomplete Hermaphroditism.
Biol. Bui., Vol. XVII, pp. 271-286, 1909.

t The whole reproductive apparatus and the dorsal part of the body
was preserved in formalin and dissected for anatomical study. The his-
tological sections of this formalin tissue showed rather poor fixation.

SIZE, SHAPE AND PHYSICAL CONSTITUTION OF THE EGG. 1 3 1

upon the deposition of yolk. The teleological view that the
oviduct enlarges in order to be ready to lay the first egg is not
tenable. Normally the oviduct enlarges while the yolks enlarge
and reaches functional efficiency by the time the first yolk is
mature.

The stimuli which initiate the peristaltic action of the oviduct
and attract the funnel to the mature folicle are among the un-
solved problems of physiology. The yolk is already oriented
in the follicle before the funnel encloses it. In pigeons * both
chalazal and polar axes are determined in the primordial folli-
cle ; the first by the long axis of the oocyte and the second by
the eccentric germinal vesicle. The polar orientation of the
egg in the follicle is due to the action of gravity after the yolk
becomes movable in the follicle (at the time of the formation
of the zona radiata). The vegetative pole is heavier than the
animal pole, hence the animal pole comes to lie beneath the
stalk of the follicle.

Due to the pressure of the inclosing funnel and to increased
internal pressure, the yolk ruptures the follicle and becomes
free in the duct. At this time the secreting activity of the duct
begins. Again the precise nature of the stimulus is not cer-
tainly known. It is normally connected with the presence oi
the yolk ; but albumen secretion followed by normal membrane
and shell formation have been induced in functioning oviducts
by the insertion of artificial yolks. Results of this kind are
reported by Weidenfeld t and TarchanoffU Unpublished inves-
tigations at this laboratory have also shown that an egg may be
formed around an artificial yolk or the artificial yolk may be
fastened in the upper part of the oviduct and an egg may be
formed which contains chalazae, both kinds of albumen, normal
membrane and shell. This last result as well as the not infr^
quent occurrence of small yolkless eggs which contain all the
other normal egg parts show that the secretory process may

* Bartelmez, G. W. Loc. cit.

t Weidenfeld. Verh. der Ornithol. Gesellschaft, Bayern, 1905, p.
1 12.

t Tarchanoff, J. R. Ueber die Verschiedenheiten des Eiereiweisses bei
befierdert geborenen (Nestfluchter) und bei nackt geborenen (Nes-
thocker.) Vogeln und fiber die Verhaltnisse zwischen dem Dotter und
dem Eiereiweiss. Pfliiger Archiv fiir Physiologie, Vol. 33, 1884.

132 MAINE AGRICULTURAL EXPERIMENT STATION. I914.

be started off by some adequate stimulus (probably in the case
of the yolkless eggs by the presence of a yolk in the oviduct
which is later expelled into the body cavity) and may then
continue to completion without the presence of the yolk. These
results with artificial yolks indicate that the stimulus may be
mechanical but the experiments of Tarchanoff § and unpub-
lished work done in this laboratory also show that an egg is not
always formed around an artificial yolk introduced into an
oviduct which is in functional condition. Until the other con-
ditions necessary for the formation of eggs with artificial yolks
are determined it cannot be certainly said that the only stimu-
lus necessary is mechanical.

That the amount of albumen secretion of the oviduct is re-
lated to the degree of the stimulation by the yolk is shown by
the significant correlation between the weight of the yolk and
the weight of the albumen. Since the secretion of the duct
does not begin until the yolk has completed its growth and
severed its connection with its follicle this seems to be the
only possible explanation of the correlation. To this evidence
that the degree of stimulation influences the amount of secre-
tion may be added that from abnormal eggs which contain
unusual large or small quantities of yolk. On the one hand are
the double and triple yolked eggs and on the other the “cock-
eggs” or “witch-eggs” which contain little or no yolk. Pearl*
(To) showed that “the relation of the observed size of the
entire egg (measured here by the weight) to the number of
yolks is very accurately described by a parabola.” He pointed
out that this indicated that the amount of albumen secreted was
related to the amount of the immediate mechanical stimulation
due to the quantity of yolk present in the oviduct.

The formation of the egg membrane is a discrete process.
As an egg passes from the albumen portion into the isthmus
as much of it as is within the isthmus is covered with mem-

§ Tarchanoff, J. R. hoc. cit.

* Pearl, R. A Triple Yolked Egg. Zoologischer Anzeiger, Bd.
XXXV, pp. 418-423, 1910.

t Coste, M. Histoire du development des corps organises. Paris,

1874.

SIZE, SHAPE AND PHYSICAL CONSTITUTION OF THE EGG. 1 33

brane.t The membrane becomes thicker during the passage
through the isthmus * probably by the addition of successive
layers of secretion.

When the egg enters the uterus it is enclosed in a firm tough
membrane and is often if not always already shaped. It com
tains the yolk with chalazae and 60 to 70 percent of its albu-
men^ . ->

Within the uterus the egg receives the rest of its albumen
(by osmosis) and its shell. The nature of the stimulus which
sets up the shell secreting activity was investigated by Pearl and
Surface.* The conclusions from their preliminary experi-
ments were that the nature of the immediate stimulus which
sets the shell secreting activity going in an oviduct in active
functional condition is mechanical, and that shell formation is
a local reflex not immediately dependent upon a specific activ-
ity of other portions of the reproductive apparatus.

That the weight of the shell is significantly correlated with
the weight of each of the other parts indicates that the amount
of secretion is influenced by the degree of stimulation. The
larger the egg the greater the mechanical stimulation and hence
the heavier the shell. The higher correlation between albumen
and shell than between yolk and shell is probably due almost
entirely to the fact that the albumen is heavier than the yolk.
It may be partly due to the fact that 30 to 40 percent of the
albumen is secreted in the uterus and that the periods of the
albumen secretion and shell secretion overlap although the
former is evidently complete before the latter has advanced
very far.f

Abnormally thin shelled eggs are as likely to be large as
small and no doubt represent either a premature expulsion of
the egg or an early arrest of the shell, secreting activity.

$ Pearl and Curtis. Studies in the Physiology of Reproduction of
the Domestic Fowl. V. Data Regarding the Physiology of the Oviduct.
Jour. Exp. Zool., Vol. 12, 1912, pp. 99-124.

'Membranes on eggs just completely past the cranial end of the
isthmus weigh .24 to .28 grams. Those at the caudal end .53 to .58
grams.

§ Pearl and Curtis. Loc. cit.

* Pearl, R. and Surface, F. M. Science, N. S., Vol. XXIX, pp. 428-
429, 1909.

t Pearl and Curtis. Loc cit.

134 MAINE AGRICULTURAL EXPERIMENT STATION. I914.

The shape of the egg is almost certainly due to the interac-
tion of the two layers of muscle fibers in the oviduct walls."
The inner layers of fibers is circular, that is they pass around
the duct. The outer layer is longitudinal and somewhat spiral
and extends into both the dorsal and ventral ligaments. Fur-
ther work on the physiology of these muscles is necessary to
determine the exact way in which they act. From their posi-
tion and from observed activities of the duct it seems that the
contraction of the circular fibers contract the duct and move
the egg forward. The contraction of the longitudinal fibers,
which have a somewhat spiral course, expand the duct, dimin-
ishing the resistance to the passage and also give the egg the
spiral motion. If the resistance is slight, i. e., if the contrac-
tions are so timed that the duct ahead of the egg is expanded
at the time of the contraction of the circular fibers behind, the
egg will be long, narrow and pointed. On the other hand, if
the resistance is great the egg will be short and broad.

The individuality of the eggs of a bird in respect to shape
must be due to an individuality in the coordination of these
two sets of muscle fibers and similarily the variation must be
due to a variation under different conditions in the degree of
coordination.

Breadth is more closely correlated with weight of the whole
egg or with the weight of any of the parts than is length. This
may be explained by the assumption that the larger the egg
(beginning with the yolk) the greater the resistance to its pas-
sage and hence the broader it will be in proportion to the actual
weight. This may be simply the mechanical effect of a larger
body passing through the elastic tube or it may be due to an
unequal increase in the effective stimulation of the two sets of
muscle fibers.

General Summary.

This paper is an analysis of the normal variations in the size,
shape, and physical constitution of the eggs of the domestic

$ 'Curtis, M. R. The Eigaments of the Oviduct of the Domestic Fowl.
Ann. Rept. 'Me. Agr. Expt. 'Sta., 1910, pp. 1-20.

Surface, F. M. The Histology of the Oviduct of the Domestic Hen.
Ann. Rept. Me. Agr. 'Exipt. Sta., 1912, pp. 395-430.

SIZE, SHAPE AND PHYSICAL CONSTITUTION OF THE EGG. 135

fowl. The data on which it is based are the measurements of
all of the eggs laid by twenty-two Barred Plymouth Rock birds.
For thirteen of these birds records were taken on every egg
laid up to the end of the second laying year. For the remainder
records were taken covering the eggs of the first laying year.
The thirteen birds which were alive at the beginning of their
second adult molt were killed at that time. The more impor-
tant results of the investigation may be summarized as fol-
lows :

i. The Individuality of the Eggs of the Same Bird.

1. The individuality of a bird is expressed in each physical
character of her eggs.

2. This individuality is more pronounced in respect to the
size than the shape of eggs and also in respect to the weight of
albumen and shell than weight of yolk.

3. There is a tendency for the several egg characters to be
related to each other in such a way that when the eggs of an
individual are large they are both long and broad and each of
the parts is large; but the hens which lay large eggs lay eggs
with a smaller proportion of yolk than hens which lay small
eggs. Also individuals may show a decided tendency to vary
from the flock type in quite different degrees in different
characters.

4. The eggs of an individual tend to be either uniform or
variable in all the egg characters but certain individuals may
be variable in certain egg characters and uniform in others.

5. An individual is in general less variable than the race in
respect to egg characters; but certain individuals may show a
variation in an egg character which is relatively as great as the
variation in the race. Also certain egg characters (particularly
yolk weight) show a decided tendency to. approach the race
variation in several individuals.

6. The factors which bring about the individuality in re-
spect to egg characters are too complex for analysis from the
data at hand.

II. Correlation of Egg Characters.

1. Each egg character is related to every other egg char-
acter, but different pairs of characters show a decidedly differ-

136 MAINE AGRICULTURAL EXPERIMENT STATION. I9I4.

ent degree of correlation. There is a general tendency for a
given pair of characters to be similarly related in the eggs of
the several individuals, but different individuals may show
significantly different degrees of correlation in any pair of
characters.

2. Length and breadth are significantly but not highly cor-
related. Both length and breadth are significantly correlated
with the weight of the whole egg and of each of the egg parts.
Breadth is as a rule more highly correlated with these weight
characters than is length. The shape of the egg as measured
by the length-breadth index is negatively correlated with the
weight of the egg and with the weight of each of the egg parts.

3. The weight of each part of the egg is positively corre-
lated with the weight of both the other parts.

III. Intra-individual Variation.

1. The variation among the eggs of the same bird is shown
to be related to certain other changes in the bird.

2. The egg weight and the weight of the egg parts, es-
pecially the weight of the yolk, increases as the bird matures.
The rate of this gain in weight decreases with the successive
months.

3. Each part of the egg shows a seasonal fluctuation in
weight which is apparently related to the general seasonal fluc-
tuation in the physiological activities of the bird, expressed
also in the curves for food consumption and egg production.

4. The state of health also may affect the size of the egg.

5. The size of the egg is related to the rate of production
as it expresses itself in the laying of litters. As a rule the first
and last eggs of a litter are smaller than the intermediate ones.

6. When eggs are produced on successive days they tend
to decrease in weight while the egg laid on a day after one on
which no egg is produced is larger than the last egg of the pre-
ceding series.

I -3S.5 Curtis, M.B.

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