396 
Journal of Agricultural Research 
Vol. XXIX, No. 
adult. Its weight increases noticeably 
in the older chickens, especially in the 
fat hens. 
8. The weight of the trachea and 
lungs is variable and shows an apparent 
sex difference in the older chickens, 
being heavier in the males. 
9. The heart shows a marked increase 
in both absolute and percentage weight 
during the later part of the growth 
period, or beginning at about 1,400 
grams gross body weight. 
10. The curve of absolute weight of 
the thyroid is concave superiorly 
throughout. The percentage values 
are variable with a minimum from 200 
to 400 grams gross body weight. 
11. The thymus increases in both its 
percentage and absolute weight up to 
sexual maturity and then undergoes 
involution, decreasing in relative and 
absolute weight. Its changes are more 
closely related to age than to body 
weight. 
12. The suprarenals and hypophysis 
are somewhat variable in weight but 
neither shows a sex difference. 
13. The kidneys show a marked 
initial rise from 0.6 per cent on day of 
hatching to 2 per cent of the body 
weight at five days, followed by a 
slow decrease to about 0.6 per cent in 
the older and adult chickens. 
14. The ovary, oviduct, testes, comb 
and wattles are extremely variable 
in weight. They all tend to form a 
four-phase curve of growth, with a 
marked acceleration at puberty. 
15. The brain, spinal cord, and eye¬ 
balls increase rapidly at first, with a 
slow growth later. The relative (per¬ 
centage) weights of these organs show 
no initial rise, but decrease progres¬ 
sively from time of hatching. 
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