GROWTH OF AMPHIBIA AFTER THYROIDECTOMY 37 



The kidneys, liver, and heart of mammals are known to vary 

 in size with the rate and amount of metabolism, when the ani- 

 mals are in a given stage of their life, and the same is probably 

 true of these organs in amphibia. The renal excretion of a 

 young frog is doubtless less in amount than that of the much 

 larger tadpole from which it is transformed, and smaller kidneys 

 are needed by the frog than by the tadpole, hence the decrease 

 in size suffered by the kidneys. The fact that the young frog's 

 kidneys are relatively larger than those of the tadpole may in- 

 dicate that the frog's renal excretion is relatively greater in 

 amount than that of the tadpole. We have made no study to 

 determine whether the loss in the size of the kidney during 

 metamorphosis is due to a decrease in the number of tubules 

 or in the length or diameter of these structures. In the thyroid- 

 less tadpoles which ultimately become from two to three times 

 as large as the control larvae, the kidneys are relatively larger 

 than those of the latter (figs. 79 and 83), and are correspondingly 

 larger than those of the smaller thyroidless larvae (fig. 80). 

 They are of about the same relative size as those of young frogs. 

 We have no data by which to compare the kidneys of older 

 frogs with those of the older thyroidless larvae. In the thyroid- 

 less larvae which were autopsied at the end of the second season 

 the kidneys were relatively smaller than in those thyroidless 

 larvae autopsied the preceding autumn when they were at the 

 end of a period of rapid growth. 



We made no observations on the fat bodies beyond noting 

 that they vary in size, do not change much, if any, during meta- 

 morphosis, and become large in the large thyroidless larvae. 



Gills and lungs. The size of the internal gills of control and 

 thyroidless larvae is proportional to the size of the body. In 

 the older thyroidless larvae the gills grow in absolute size much 

 larger than they ever are in normal larvae and persist as long 

 as the larvae are kept alive. Thus, structures which normally 

 exist during only a few weeks or months are maintained more 

 or less indefinitely. 



The lungs normally become functional by the time larvae are 

 half-grown. The larvae can be seen to start deep in the aqua- 



