542 



W. W. SWINGLE 



of organs are transformed with startling rapidity from the larval 

 condition to that characteristic of the adult, and yet in the 

 midst of such somatic transformations, the gonads and germ 

 cells remain unaffected. 



In a state of nature it requires two seasons and sometimes 

 three, for the larvae of Rana catesbiana to reach the adult 

 condition. It is possible by judicious thyroid feeding, to bring 



Fig. 14 Photograph of starved thyroid-fed larvae and their controls. The 

 animals in the center are the thyroid-fed tadpoles. 



about almost complete metamorphosis within a period of three 

 weeks in the immature larvae of this species of frog. The 

 animals have all the body characteristics of the metamorphosed 

 frog, yet the germ cells and germ glands are those of young 

 larvae. 



In conclusion I wish to acknowledge my indebtedness to Dr. 

 B. M. Allen of the University of Kansas for suggesting this prob- 

 lem and for the interest he has shown in the work. 



