386 
Journal of Agricultural Research 
Vol. XXiX, No. 8 
roids. These and the thyroid glands 
from the three hens at 1,920, 2,230 
-and 2,440 gm. gross body weight were 
all excluded in the averages for the 
curve, although the individual cases 
are shown on the chart. There is ap¬ 
parently a sex difference in the weight 
of the thyroid in the six older chickens, 
but this may be due to a difference in 
diet. The three hens were taken from 
one of the pens of the regular laying 
stock, but the cockerels were from a lot 
of male birds isolated all winter, from 
which breeding cockerels had been 
selected. The larger birds, both male 
and female, from these pens were 
selected in order to extend the upper 
ond of the curve based on gross body 
weight. 
The percentage weights of the thy¬ 
roid plotted on gross body weight are 
exceedingly variable. The lighter line 
base of the neck or- nearer the thorax. 
The lobes are usually of a pinkish color, 
although some were much darker. 
Figure 22 gives the absolute and 
relative weights of the thymus plotted 
against age in days. The formulas are: 
7=0.1 [(o.iX)i- 6 ] 
from 10-130 days, 
7 = 0.1 [(O.lXj^-O.lSS (X-130) 
from 130-260 days. 
7 represents the weight of the 
thymus in grams and X represents 
the age in days. 
The weights of the thymus plotted 
on gross body weight do not give 
nearly so regular a curve as that 
according to age, shown in Figure 22. 
The first or growth phase in each of 
the two curves is very similar, but the 
Fig. 22.—Absolute and relative or percentage weights of the thymus, plotted on age in days 
represents the averages of the per¬ 
centage weights of the thyroid but not 
the individual cases. There is no 
apparent sex difference until the upper 
end of the curve is reached. The curve 
shows an initial decrease from 0.013 
per cent to nearly 0.009 per cent at 
300 gm. gross body weight. It then 
rises to about its initial value at 1,100 
gm. body weight, remaining at about 
this level up to 1,800 gm. of gross body 
weight, when it again rises. This last 
rise is caused by the larger percentage 
values in the adults. When the rela¬ 
tive weights are plotted on age there 
is this same depressed portion of the 
curve falling between 40 to 60 days. 
Thymus. —The thymus of the chicken 
is located farther cephalad than the 
thyroid. It consists of a chain of 
lobes lying along each side of the neck, 
from the larynx down to the thyroid 
gland. The larger lobes, which persist 
longer when the gland begins to 
atrophy, are as a rule located at the 
position of the older pullets, toward 
the center of the chart based on body 
weight, produces an irregular and more 
abrupt second or involution phase of 
the curve. Evidently the thymus in¬ 
volution in the chick depends on age 
rather than on body weight, as was 
found by Hatai (6) for the rat. 
The thymus of the chicken follows 
the usual course of development for 
the gland as found in other animals. 
It increases slightly more rapidly than 
the body up to 130 days, and then it 
decreases to nearly the same absolute 
weight as that of a chick about 40 
days old. The relative weights in the 
older chickens are naturally much less. 
At first the thymus forms a little less 
than 0.3 per cent of the net body 
weight. This percentage rises until 
at 110 days it forms about 0.46 per 
cent; then there is a decrease until it 
reaches about 0.05 per cent for the 
older chickens of this series, and slightly 
less for the adult chickens. 
