Oct. is, 1924 Postnatal Growth of the Single-Comb White Leghorn 
373 
all show an earlier maximum than do 
the curves of gross body weight (fig. 1), 
doubtless due to complete ossification 
of the skeleton before adult weight is 
attained. 
Head weight. —-The increase in ab¬ 
solute weight of the head plotted on 
gross body weight and the percentage 
of the net body weight are shown in 
Figure 7. The formulas used in draw¬ 
ing these curves of growth in absolute 
weight of the heads of the males are: 
y=x°- 5 —3 
from 50 to 300 gm. gross body weight, 
Y = X 0 -*- 0.013X-12.72 
in grams. The cases in which the 
body weight is over 2,200 gm. are 
the adult specimens. They are not 
included in the curve, although the 
individual head weights are indicated 
on the chart. From 300 gm. gross 
body weight, or 55 days of age, there 
is a sex difference in head weight, due 
in large part, as suggested above, to 
the greater development of the comb 
and wattles in the males (fig. 10). 
The percentage weight of the head 
shows no initial rise but drops rapidly 
at first and later shows a sex difference. 
Jackson and Lowrey ( 9 ) found that 
in the postnatal growth of the rat the 
head at first increases more rapidly 
Fig. 8.—Weight of the integument (without feathers) plotted on gross body weight 
from 400to 1,400 gm. gross body weight, 
and, 
Y = 0.0554 (X-1400)+ 46.28 
from 1,400 to 2,300 gm. gross body 
weight. 
The formulas for the females are: 
Y — A" 0 * 5 —3 
from 50 to 600 gm. gross body weight, 
0.0167 (X — 600) + 21.49 
from 600 to 2,500 gm. gross body 
weight. 
In all these formulas Y represents 
the weight of the head in grams and 
X represents the gross body weight 
than the rest of the body and there 
is no sex difference. The head is 
relatively much lighter in the chicken, 
for its maximum percentage weight 
in the chicken is a little less than 
half that for the rat. In the adult 
rat the head forms about 9 per cent 
of the body, while in the chicken it is 
only about 4 per cent in the male and 
between 2 and 3 per cent for the female. 
GROWTH OF THE SYSTEMS AND 
ORGANS 
INTEGUMENT 
Skin. — Figure 8 shows the weight 
of the integument, excluding feathers. 
