Oct. i5,1924 Postnatal Growth of the Single-Comb White Leghorn 
371 
23.57 cm. For the females the for¬ 
mula applies from 5 to 105 days and 
then the curve remains constant at 
20.92 cm. In this case the time of 
the cessation of growth in length is 
also less definitely marked. The ad¬ 
ult wing length in the male is reached 
at 130 days, or 11 days earlier than the 
adult leg length, and at 105 days in 
the female, which is about the same 
time that the maximum leg length is 
attained. Thus the pullets show an 
earlier development in both these 
measurements. 
Thorax. —In Figure 5 the two diam¬ 
eters of the thorax are plotted against 
age. The formula from which the 
curves for the dorsoventral diameter 
were drawn is: 
Y = X 0 - 45 + 0.014X + 0.44 
in which Y represents the dorsoventral 
diameter in centimeters and X repre- 
two sets of curves, the transverse diam¬ 
eter increases for a much longer time 
than does the dorsoventral diameter. 
This measurement is affected more by 
the deposit of fat, a large mass of sub- 
dermal fat being found on either side 
of the breast in the fat chickens. An¬ 
other factor which influences these 
measurements is the possible increase 
in size of the thoracic cavity and the 
dilation of the bony thorax. This 
measurement and the body length are 
the only two which continue to increase 
throughout the entire series and both 
of them involve other factors than 
skeletal growth. The six older chickens 
differ from the others more in these 
two measurements than they do for 
the preceding measurements shown in 
Figures 3 and 4. 
The thoracic index 
dorsoventral di amet er X 100 
transverse diameter 
Fig. 5 .—Dorsoventral diameter (upper part of chart) and transverse diameter of the thorax (lower part of • 
chart) plotted on age in days. Both diameters (in centimeters) were made just posterior to the anterior 
end of the sternum. The dorsoventral diameter appears smaller in the older females 
sents age in days. For the males 
this formula applies from 10 to 150 
days and then the curve remains 
constant at 12.06 cm. For the females 
the curve was drawn from the formula 
from 10 to 125 days, after which this 
curve remains constant at 10.97 cm. 
The formula for the transverse diame¬ 
ter of the thorax is: Y — in which 
Y represents the transverse diameter 
of the thorax measured in centimeters, 
and X represents age in days. This 
formula applies from 10 days of age 
to the adult stage. The dorsoventral 
diameter was taken just back of the 
anterior end of the crest of the ster¬ 
num and the transverse diameter was 
always taken in the same plane. 
Both of these diameters show a longer 
period of increase than do the preced¬ 
ing linear measurements. This may 
be explained by the fact that these 
measurements are not purely skeletal. 
As can be seen by a comparison of the 
99185—25f-2 
was determined for the chickens. 
Throughout the entire series, and 
including the six older birds, there 
appears no significant change and no 
sex difference. The index averages 
about 175. 
Had these two diameters of the body 
been taken in the abdominal region 
there would have been apparent a 
much more noticeable sex difference; 
for in the later part of the period during 
which the birds were studied there is 
a marked difference in the shape of 
the body in the male and female. In 
the male the maximum depth or 
dorsoventral diameter is but a centi¬ 
meter or two caudad to the point used 
in measuring this diameter. In the 
pullets, however, the anterior end of 
the body had the smaller dorsoventral 
diameter. In other words, the pos¬ 
terior region of the female body cavity 
becomes distended with the larger 
reproductive tract, and the greater 
