GROWTH AFFECTED BY FEEDING DUCTLESS GLANDS 337 



22. Summary 



a. Comparison of controls with established norms for body 

 weight, body and tail length, and size of organs. In regard to 

 body weight, the control rats used in this investigation as well 

 as the experimental animals can be divided into two groups: 

 (1) an older group ('summer-born') which were born in the 

 summer and earlj^ fall; and (2) a younger group ('winter-born') 

 which were born in the winter and spring. The first group 

 maintained about the same size and body weight as the rats 

 of Donaldson ('06) and Jackson ('13). The second group, 

 especially the males, were considerably larger at every stage, 

 and more nearly resembled in growth the selected strong and 

 vigorous litters of rats recently studied by King ('15). 



The ratio of tail length to body length is somewhat less in 

 my series than in the norms of Jackson ('15) and those calculated 

 from Donaldson's Wistar tables. That is, my rats were rela- 

 tively short-tailed. 



The variability of growth in body weight, body length and 

 tail length is thus emphasized. Such differences may occur 

 in different 'strains' of rats, especially under different conditions 

 of environment, diet, etc. Experience shows that the average 

 body weight may be considerably increased by unusual care 

 and liberal feeding. Exercise has also been shown to affect 

 markedly the growth of the body (Slonaker '12) and organs (Hatai 

 '15) of the rat. 



As to the absolute weights of the individual organs and parts, 

 the data for the controls in general agree fairly well with those 

 of Donaldson's Wistar tables for corresponding body length 

 and body weight (tables 4 and 5). The differences are in most 

 instances no greater than might be expected from normal varia- 

 bility, especially in rats of different strain kept under different 

 environment. In some cases (e.g., thyroid gland, thjinus, and 

 alimentary canal) there are differences due to difference in the 

 technique of remo\'ing the organs. 



In the case of the very rapidly growing rats, it is quite probable 

 that some of the peculiarities (e.g., of skeleton, spleen, liver, 



