Oct. 15, 1924 
Postnatal Growth of the Single-Comb White Leghorn 
395 
than does the male skeleton. A similar 
difference in extent of growth is true 
for other organs, but this does not 
necessarily mean that these organs are 
proportionally heavier in the male of 
the same body weight. Plotting these 
organs against gross body weight 
usually shows clearly that they are 
relatively of about the same weight in 
male and female. As has been men¬ 
tioned, there still persists a definite 
sex difference in the weights of the 
head, skeleton, and feathers and a less 
marked sex difference in the heart and 
lungs. The digestive system and all 
of its parts (except the liver), and the 
nervous system and eyeballs show a 
slight sex difference in the upper parts 
of the growth curves for these organs. 
By the time this sex difference be¬ 
comes apparent, the chickens are 
sexually mature and the females are 
all fatter than the males. The extra 
fat thus increases the total body weight 
perhaps enough to account for the 
apparent lowering of the weights of 
the organs in the females, when plotted 
against body weight. The liver does 
not follow the same course as the other 
parts of the digestive tract, for it also 
apparently becomes loaded with the 
surplus food material and the fat 
females therefore also have heavier 
livers. This is readily seen by refer¬ 
ence to Figure 17. 
If, however, the surplus fat in the 
females makes the digestive system 
and the nervous system appear rela¬ 
tively lighter in the females, it should 
affect all of the organs similarly. But 
the suprarenals, thymus, spleen, skin, 
and kidneys do not show this effect. 
It is true that most of these curves are 
more irregular, but at least some 
indication of the sex difference in 
weight might be expected, unless 
some other factor is concerned in this 
group of organs. 
The poultrymen say that chickens 
two years old should be heavier than 
at one year of age. Although the 
writer’s cases are very few, it may be 
interesting to see what changes are 
apparent in the structure of the older 
chickens. An actual increase appears 
in the weights of the kidneys of two of 
the three hens. The heart shows an 
increase in absolute and in percentage 
weight. The thyroid becomes ex¬ 
tremely variable, heavier for one and 
much lighter in two of the females. 
The amount of fat was not determined 
accurately, but from inspection it 
is apparent that a very large part of 
the change occurring in the second year 
is due to the increase in the amount of 
fat. There is an absolute increase in 
99185—25f-3 
weight of the muscles (possibly due to 
more fat within them), though they 
still maintain the same relative (per¬ 
centage) weight. 
SUMMARY 
The more important findings may be 
summarized as follows: 
1. The curve of postnatal growth of 
the entire body of the chicken shows 
three general phases: A period, first, of 
slow growth, including a brief post¬ 
natal decrease in weight; a period, sec¬ 
ond, of rapid (pubertal) growth, during 
which a sex difference in body weight 
begins; and a period, third, of slow 
increase in weight. 
2. The weight of the head shows a 
marked sex difference, due apparently 
to the larger development of the comb 
and wattles in the male. 
3. The growth in weight of the skin 
(excluding feathers) is directly propor¬ 
tional to that of the entire body, form¬ 
ing about 9 per cent of the net body 
weight. 
The feathers increase in both abso¬ 
lute and relative weights until just be¬ 
fore sexual maturity. Then follows a 
decrease in absolute and relative 
weights, the growth curve thus some¬ 
what resembling that of the thymus. 
4. The skeletal muscles increase from 
21 or 22 per cent at hatching to about 50 
per cent of the body weight in the 
adult. 
5. The skeleton at first grows a little 
less rapidly than the entire body. It 
forms 11 per cent of the body weight 
in the mature male, and 8 per cent in 
the female. The weights and linear 
measurements show that the female 
skeleton matures earlier than the male. 
6. The digestive tube and its regions, 
stomach (proventriculus), gizzard, and 
intestines, also the pancreas, all grow 
at about the same rate, showing a short 
initial rise in relative weight, followed 
by a slow decrease up to maturity. 
The empty tube reaches a maximum 
of 18.5 per cent of the body on the sixth 
day, decreasing to about 5 per cent in 
the adult. 
The weight of the “tare” (contents 
of the digestive tube) is extremely vari¬ 
able. 
The yolk sac was found in all chick¬ 
ens autopsied, with one exception, up 
to and including the thirty-eighth day, 
and thereafter frequently up to the 
two hundred and thirty-seventh day. 
Meckel’s diverticulum is constantly 
present. 
7. The liver decreases from an early 
maximum of 6.2 per cent of the body 
weight to about 2.5 per cent in the 
