48 E. R. HOSKINS AND M. M. HOSKINS 



thymus is relatively larger than in smaller thyroidless or control 

 larvae. 



The epithelioid (parathyroid) bodies are not affected by thy- 

 roidectomy until after metamorphosis of the controls. They 

 become relatively large in large thyroidless larvae. They in- 

 crease in relative size during metamorphosis. 



The thyroid is necessary for metamorphosis of frog larvae kept 

 on normal diet. The thyroid of normal animals increases in 

 relative size during metamorphosis, but shows no actual change. 

 The thyroid is necessary for full ossification of the skeleton, but 

 rriay not be necessary for the atrophy of tissues that occurs during 

 metamorphosis. Excessive amounts of thyroid hasten metamor- 

 phosis, but a th3T:-oid much smaller than normal is sufficient to 

 permit metamorphosis. 



The kidneys become relatively large in thyroidless larvae. 

 During metamorphosis in normal animals the kidneys decrease 

 in size, but less than does the body as a whole. 



The spleen does not appear to be directly affected by thy- 

 roidectomy, but it becomes large in large larvae. The variability 

 in the size of the spleen is very great. The spleen does not 

 appear to change in actual size during metamorphosis, but in- 

 creases in relative size. 



The internal gills persist in animals kept in the larval condition 

 by thyroidectomy. The lungs develop and become functional 

 in both normal and thyroidless larvae. 



The intestine becomes long in large thyroidless larvae and 

 retains its larval character. 



The ovaries become large in thyroidless larvae, and oocytes 

 develop in them. The growth and differentiation of the ovary 

 are to a limited extent independent of the growth and differ- 

 entiation of the body as a whole. The oviducts do not develop, 

 at least during the first year and a quarter. Maturation did- 

 not occur in the ovaries of thyroidless larvae in our experiments. 

 The ovaries of normal animals are not directly affected by 

 metamorphosis. 



The testes become fully mature in thyroidless larvae, producing 

 spermatozoa which escape into the kidneys. Young frogs have ^ 



