530 



W. AY. SWINGLE 



differences either in regard to the size or development of the 

 glands or germ cells (figs. 4 and 5). Measurements of the 

 spiral gut of the two groups of larvae, as was noted for earlier 

 series examined, revealed remarkable atrophy of the alimentary 

 tract of the thyroid-fed larvae. In these animals, the typical 

 frog gut had replaced the simple, tube-like digestive tract of the 

 larval form. 



No further work was done along this line, until the late summer 

 and early Fall of 1915, when feeding experiments were carried 



f 



? 



I 



Fig. 3 Photograph of thyroid fed and control larvae. The large tadpoles 

 are the controls. The black color of the experimental animals is partly due to 

 their having been fixed in Flemming's solution. 



out with Rana catesbiana and one group of larvae, the species 

 of which has not been determined. I shall discuss the experi- 

 ments upon the unidentified larvae first, because their growth 

 rate more nearly resembled that of Rana pipiens. 



November 17, 1915, a new series of very young larvae were 

 started upon the thyroid diet. These animals, eighty in num- 

 ber, were found in a shallow pool near the University, and, judg- 

 ing from the size of the pool which was hardly more than 2 feet 

 wide, and of the larvae, which varied but a few millimeters in 

 length, appeared to be from the same batch of eggs. The 



