;394 
Journal of Agricultural Research 
Vol. XXIX, No. 8 
cord. There is the same indication of 
a sex difference (of doubtful signifi¬ 
cance). The formula from which this 
curve was drawn is: 
Y = X 0 - 251 —1.91 
from 100-2,600 gm. gross body weight. 
Y represents the weight in grams of 
the two eyeballs and X represents the 
gross body weight in grams. 
Welcker and Brandt (38) found a 
relative weight of 0.30 per cent for 
the eyeballs in the chicken, which is a 
little higher than found in the present 
series. The three older Minnesota 
cockerels averaged 0.22 per cent, and 
the hens 0.21 per cent. The maximum 
relative weight of the eyeballs in the 
chick comes at the time of hatching, 
with a rather strikingly larger per¬ 
centage weight for the males during 
the first few days. 
DISCUSSION 
The growth of the body as a whole 
in the chicken seems to be somewhat 
different from that described in man 
by Scammon (22) and Morris. 5 The 
general character of the curve of growth 
in the writer’s series is similar to that 
found in earlier investigations on the 
chicken. This curve shows three phases: 
(1) a period of slow growth, which in¬ 
cludes a brief period of postnatal de¬ 
crease; then (2) a period of rapid growth, 
followed by (3) another period of slow 
growth. This general type of curve 
holds in the chicken not only for the 
growth of the entire body but also for 
the muscles, ligamentous skeleton, 
digestive tract, lungs, heart, kidneys, 
suprarenals and integument, when their 
• weights are plotted against age. In 
man, the growth curve of the body as 
a whole and of all of the above parts, 
excepting the suprarenals, is a four- 
phase curve. The suprarenal in man 
shows, according to Scammon— 
a great decrease in weight in the neonatal period, 
an interval from the neonatal period which extends 
through the greater part of childhood when there 
is little growth, and a period of rather rapid growth 
in the prepuberal period and in adolescence. 
There is in the chick but slight in¬ 
dication of a slowing of growth, 
corresponding to the decreased rate of 
human growth in middle childhood. 
It is very doubtful whether anything 
corresponding to the characteristic 
human prepuberal acceleration of 
growth occurrs in the chick body as 
a whole. 
When the weights of the organs and 
systems are plotted against gross body 
weight instead of age, the first period 
of slow growth is usually not observed, 
as shown in the various charts. The 
exceptions to this are the curve for the 
thyroid, which is distinctly concave 
upward throughout its entire length; 
also those for the muscles and integu¬ 
ment, which are nearly straight, and 
those for heart and lungs. 
Another type of curve described by 
Scammon (22) for the growth in the 
weights of the human brain, spinal 
cord and eyeballs is similar to the 
curves for the same organs in the chick 
when the weights are plotted on either 
age or gross body weight. All of these 
curves rise rapidly at first and then 
more slowly flatten out. 
The type for the human thymus 
with an initial rise up to the age of 
puberty followed by a decrease in 
weight is, according to Hammar, 
characteristic for mammals in general. 
The present study shows that it holds 
likewise for the chick. It is also the 
type of curve found for the growth of 
the feathers, a fact apparently hitherto 
overlooked. As above explained, how¬ 
ever, the decrease in the weight of the 
feathers is due to a process quite 
different from the involution of the 
thymus. 
The growth of the testes of the chicken 
shows (1) an initial period of but little 
change; (2) a slightly increased rate of 
growth followed by (3) a second period 
of slow growth and of marked irregu¬ 
larity, then a period of very rapid 
prepuberal growth and finally (4) a 
period of slow growth. The cases are 
too few to make a smooth curve but 
they indicate the above four phases. 
This is similar to the curve of growth 
for the human genital organs, excepting 
ovary and uterus. 
The ovary and oviduct of the chicken 
seem to grow slowly until the very 
rapid prepuberal growth, after which 
there is a period of but little growth. 
The oviduct is lighter than the ovary 
up to the prepuberal rise. The num¬ 
ber of cases is insufficient to warrant 
final conclusions concerning the growth 
of the ovary and oviduct, and a com¬ 
parison of their growth with that in 
other forms is therefore omitted. 
The curves of the growth of the body 
as a whole show a sex difference 
apparent very soon after the begin¬ 
ning of the second or rapid puberal 
growth phase of the curve. This 
means that at any age after 8 weeks 
the male chickens average heavier 
than the females of equal age. 
The linear measurements of the bod}^ 
also show a marked slowing of growth 
in the female earlier than in the male. 
This means that the female skeleton 
ceases to increase in length earlier 
5 Morris, II. morris’s human anatomy. Ed. 6, Sect. 1. 1921. 
