Oct. 15,1924 Postnatal Growth of the Single-Comb White Leghorn 377 
difference. There is no postnatal de¬ 
crease in percentage weight of muscle, 
as described, by Jackson and Lowrey 
(9) for the rat. 
The relative weight of the muscular 
system seems to vary greatly in differ¬ 
ent species. According to Welcker 
and Brandt {28) it varies from about 
19 per cent in the tortoise to nearly 59 
per cent in the perch. Their average 
for the domestic fowl was 54.5 per cent. 
Jackson and Lowrey ( 9 ) have pointed 
out that the percentage weight of the 
muscles does not vary in proportion to 
the size of the animal. There seems 
to be some evidence to indicate that 
within a group or phylum of animals, the 
activity, or the ability to perform pow¬ 
erful or rapid movements, is correlated 
with the relative weight of the muscu¬ 
lature in the animal. 
Y represents the weight of the moist 
ligamentous skeleton in grams and X 
represents the gross body weight in 
grams. 
This curve shows no sex difference 
until a body weight of 900 gm. is 
reached. Unfortunately, there are no 
male skeletons at body weights be¬ 
tween 950 and 1,190 gm., but there is 
probably no marked change, since the 
curve for the males continues in a 
straight line up to 1,200 gm gross body 
weight. 
At about 900 gm. of gross body 
weight there comes a marked change in 
the curve for the female skeletons; it 
continues to increase but not nearly so 
rapidly as before this time. The curve 
for the male skeleton continues at the 
same rate until 1,200 gm. in body weight 
is reached. From this point on, the 
Fig. 12—The curves represent the absolute weights (heavier line) and the percentage weights (lighter line) 
of the moist ligamentous skeleton, plotted on gross body weight 
SKELETAL SYSTEM 
Figure 12 shows the absolute and 
the percentage weights of the moist 
ligamentous skeleton, plotted against 
f ;ross body weight. The formulas for 
he males are as follows: 
Y= 0.14X 
up to 1,200 gm. gross body weight, 
Y= {X -1100) 0 - 7 - 0.04X+190.88 
from 1,200-2,500 gm. gross body 
weight. 
For the females the formulas are: 
F=0.14X 
up to 900 gm. gross body weight, 
Y= (0.01A) 1 * 7 —0.075A +151.64 
from 900-2,400 gm. gross body weight. 
male, too, shows a relatively slower 
skeletal growth. 
The cases above 2,200 grams in gross 
body weight shown on the chart are 
the six older chickens. It is seen that 
these fit into the curve for the younger 
chickens, indicating that there is no 
radical change in the skeletal weight 
after about 300 days. When the 
weights of the moist ligamentous 
skeleton are plotted against age there 
is even a more evident sex difference, 
the curves of male and female skeleton 
separating at about 120 days of age. 
This appearance of a sex difference in 
weight of the skeleton corresponds 
rather closely to the beginning of a sex 
difference in the growth of the linear 
measurements of the charts shown in 
Figures 3 to 5. 
