METAMORPHOSIS IN FROG LARVAE 



531 



average length of the tadpoles was 17 mm. Fifty of these larvae 

 were selected for thyroid feeding purposes, and divided into 

 lots of twenty-five each, one group to serve for controls. Both 

 groups were placed under identical environmental conditions. 

 The first administration of thyroid was on November 17. 



All of the body changes, characteristic of thyroid-fed larvae, 

 soon appeared, and by November 25th, eight days after the first 



Fig. 4 Representative transverse section through the gonad of thyroid-fed 

 animal in advanced stage of metamorphosis. X 800. 



Fig. 5 Transverse section through the gonad of control for animal shown in 

 figure 4. X 800. 



administration of the thyroid, limb buds of the posterior ex- 

 tremities were clearly discernable. When measured, the thy- 

 roid-fed larvae still retained their length of 17 mm.; thus indi- 

 cating that growth had ceased when the thyroid feeding began. 

 Other features characteristic of hyperthyroidism observed at 

 this time were: protrusion of the eyes, elongated appearance of 

 the head, atrophy of the tail, and increased pigmentation. 



The controls of this group of larvae, averaged 20.5 mm. in 

 length, showing an increase in length of 3.5 mm. during the 



