METAMORPHOSIS IN FROG LARVAE 



529 



continued until May 25th. Upon this date, all of the thyroid- 

 fed larvae remaining alive were killed and preserved for section- 

 ing. They averaged in length 20.5 mm. The tails of most of 

 them were hardly more than a mere stub. The fore and hind 

 limbs were well developed; in fact metamorphosis was almost 

 complete. The .young frogs, however, were very small. 



Fig. 1 Representative transverse section through the gonad of a thyroid- 

 fed larvae in advanced stage of metamorphosis. Fed thyroid fifteen days. D, 

 follicle cell; B, secondary genital cavity; C, germ cell. X 800. 



Fig. 2 Transverse section through the gonad of the control for animal de- 

 scribed in figure 1. D, follicle cell; B, germ cell; C, mesenchyme cell. X 800. 



The control animals for this series showed none of these 

 changes but instead, had doubled their length, averaging 41.5 

 mm. There were no indications of metamorphosis other than 

 the tiny limb buds of the posterior extremities. These buds 

 usually make their appearance when the animal is about 25 mm. 

 in length. The limbs, however, had not differentiated into their 

 two primary divisions, nor had the toes appeared (fig. 3). 



Microscopic examination of the gonads and germ cells of 

 both control and thyroid-fed larvae revealed no observable 



