The Development of the Frog 57 



metamorphosis, when the circulation changes 

 from practically that of a fish to that of the 

 adult frog. Since changes similar to these 



H.B. 



Fig. 21. — Transverse section through the re- 

 gion OF THE HIND-BRAIN OF A YOUNG TADPOLE. 



Z*, digestive tract. E, ear vesicle. H B^ hind-brain. Ny 

 notochord. S^ sucker. (Camera lucida.) 



take place in even the highest animals, they 

 are of more than passing interest. 



The development of the heart and pericar- 

 dium, though difficult to follow out in detail in 

 the laboratory, will be readily understood from 

 the following description, studied in connection 

 with Fig. 25 : 



