12 E. R. HOSKINS AND M. M. HOSKINS 



One 1917 operated larva deserves special mention. Its thyroid 

 regenerated and then hypertrophied. The animal metamor- 

 phosed about a month before the controls and after metamorpho- 

 sis it was less than one-third the normal size (fig. 20). The 

 length of the controls at this time was 40 mm. (fig. 21). An 

 attempt was made to duplicate this experiment in 1918 by cutting 

 to pieces the thyroid anlage and leaving the pieces in the animal. 

 In other specimens transplantation of the thyroid was performed, 

 but in neither group did these experimental larvae metamorphose 

 .earlier than the controls, although a few were smaller than nor- 

 mal. In three 1918 specimens from which the thyroid had been 

 removed, the gland regenerated and the animals metamorphosed 

 at a size about one-half that of the average control larva, but 

 not at an earlier date. 



Figures 10 to 19 show the changes in body form exhibited by 

 the control and thyroidless animals. Figures 12 and 13 show 

 a normal tadpole in the process of metamorphosis. The hind 

 legs are well developed and the skeleton of the body has begun 

 to acquire the adult shape. In figure 12 it may be noted that 

 the head is flattening and narrowing. The larva shown in fig- 

 ures 14 and 15 is a thyroidless specimen of the same age as that 

 of figures 12 and 13. Its total length is greater than that of 

 the control larva, the hind legs are very small and the body 

 (fig. 14) is more nearly cylindrical than that of the control (fig. 

 12). The eyes are more laterally placed in the control larva 

 and, although its head is narrower than that of the other (fig. 

 15), the eyes are further apart. The head of the control speci- 

 men is slightly more pointed than that of the thyroidless larva 

 and is more like that of the frog (fig. 17). The experimental 

 animal retains this larval form for some time, but it slowly under- 

 goes changes in shape which tend toward those of the controls. 

 Normally, during the period of metamorphosis, the form of the 

 animal changes as follows: The body decreases very slightly in 

 length; the anal canal shortens 2 to 4 mm. (compare figs. 12 and 

 16); the transverse and horizontal diameters decrease nearly 

 one-half; the tail atrophies; the dorsal and ventral sides flatten; 

 the eyes come actually closer together, but are relatively farther 



