387 
Oct. 15,1924 Postnatal Growth, of the Single- Comb White Leghorn 
The gross appearance of the in¬ 
volution changes indicate that the 
usual two methods of obliteration are 
found in the chicken, namely, a fatty 
or a fibrous involution. The lobes 
become smaller and later a decrease in 
the number of lobes is apparent. 
Throughout the entire period more or 
less fat was found surrounding the 
thymus and the lobes were enclosed 
by the fascia. 
Jackson (8) found the following rela¬ 
tive weights for the thymus in the 
male white rat: at birth it forms 0.15 
per cent, which makes it slightly 
lighter than that of the chick; then it 
increases to a maximum of 0.38 per 
cent at 20 days, and again decreases 
to 0.02 per cent at one year. Since 
the rat matures more rapidly than the 
Accessory spleens were noted in 4 
specimens (2 males, 2 females), ranging 
in age from 54 days to 237 days, or 
from 420.6-2,023.5 gm. in gross body 
weight. They were found in the neck, 
at the anterior end of the crop, close 
to the hilum of the spleen, and in the 
237-dav female, near the pancreas on 
the opposite side from the normal 
spleen. 
Suprarenal glands. —Figure 24 
shows the absolute and relative growth 
of the suprarenal glands plotted against 
gross body weight. The formula is: 
F=A 0 - 012 + 0.000077X -1.046 
from 100-2,600 gm. gross body weight. 
Y represents the weight of the supra¬ 
renal gland in grams and X, the gross 
body weight in grams. 
Fig. 23.—Absolute and relative or percentage weights of the spleen, plotted cn gross body weight 
chicken, this earlier maximum in the 
growth of the thymus is to be expected. 
Spleen. —Figure 23, which gives the 
absolute and relative weights of the 
spleen plotted against gross body 
weight, shows that the spleen is ex¬ 
tremely variable in the chicken, as 
it has been found to be in other ani¬ 
mals. The formulas are: 
F=X°- 12 + 0.00132X-1.74 
from 100-1,200 gm. gross body weight, 
7=0.00039 (X- 1200) +2.18 
from 1,200-2,600 gm. gross body weight. 
Y represents the weight of the spleen 
in grams and X, the gross body weight 
in grams. 
The relative weights show an initial 
increase followed by a slow decrease 
after 1,100 gm. in gross body weight. 
Some of the irregularity in the upper 
part of this curve may be due to the 
difficulty in removing these glands. 
In the female the left suprarenal is 
lodged at the base of the ovarian liga¬ 
ment, and when the ovary is fully de¬ 
veloped the ligament is so strong and 
tough that the removal of the left 
suprarenal gland without injuring it is 
difficult. 
There is possibly a very slight in¬ 
crease in the percentage weights of 
the suprarenal after hatching, with a 
maximum value for one specimen of 
0.0359 per cent on the fourth day. 
Then there is a decrease until the 
average for the adult is about 0.01 per 
cent. These values are slightly greater 
for the maximum and a little less for 
the adult than given by Jackson («?) 
for the male rat. 
