GROWTH AFFECTED BY FEEDING DUCTLESS GLANDS 327 



identical conditions. According to Jackson ('13) the coefficient 

 of variability in weight of the thymus at 10 weeks is 22 to 25. 



When compared in absolute weight with Donaldson's Wistar 

 norm for rats of corresponding age (tables 4 and 5) it appears 

 that in my rats the thymus is somewhat lighter, except in the 

 younger male group. The age involution is well known on 

 comparing the older and younger groups (tables 6 to 9), the 

 corresponding body weights in both cases being not greatly 

 different. 



b. Thyroid groups. The thymus shows no constant differ- 

 ence in weight that can be attributed to the thyroid diet. It is 

 under weight in the females by an average of 10 per cent, but not 

 in the males. 



c. Thymus, hypophysis, and pineal groups. The thymus 

 appears relatively much heavier than normal in the thymus- 

 fed older males, but as no corresponding difference is found in 

 the younger males, or in the females, the result is probably due 

 to accidental variations. 



10. Heart (tables 4 to 10) 



a. .Controls. In absolute weight, the heart of the older rats 

 in my series (tables 4 and 5) is in general in fairly close agreement 

 with the corresponding data in Donaldson's tables, but in my 

 younger males the heart is considerably heavier than Donald- 

 son's norm for rats of corresponding body weight or length. 

 Jackson ('13) also found the normal heart relatively somewhat 

 heavier than would be expected according to Hatai's curve of 

 theoretical growth. 



b. Thyroid groups. The heart shows a very marked hyper- 

 trophy in rats to which thyroid was fed, excepting 3 old males, 

 in which the dosage was too small to be effective. If calculated 

 by Donaldson's method, the hypertrophy amounts to 24.6 and 

 16.7 per cent for the older and younger females respectively, 

 and 36 and 15.4 per cent for the corresponding males (table 10). 



c. Thymus, hypophysis, and pineal groups. No constant 

 or apparently significant variations appear in the heart in these 

 groups. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 21, NO. 3 



