CLASS AMPHIBIA 



379 



tion called the stomodeum marks the beginning of the mouth, while 

 toward the posterior end of the body, below the tail, which is developing 

 backward, is formed another invagination, the proctodeum, which will 

 become the cloacal opening. The medullary groove is converted into a 

 medullary tube by the meeting of the ridges on each side of it. This 

 tube in turn develops into the central nervous system. Eyes appear 

 on each side of the head, and external gills are formed which project 

 outward from the branchial arches. At the same time muscle segments 



Oral sucker 



Fig. 268. — Later stages in the development of the frog. A, embryo at time of hatching. 

 B, tadpoles clinging to vegetation after hatching. C, stage showing external gills. D, 

 gills covered by an operculum, the branchial chamber opening to the outside by the spiracle; 

 hind legs appearing. E, hind legs well developed. F, late stage in metamorphosis; legs 

 all present, and tail nearly gone. G, the adult leopard frog, Rana pipiens Schreber. From 

 models, and preserved and living (Fig. G) specimens. 



are seen developing under the skin on each side of the body and tail. 

 The yolk is massed in the ventral portion of the body causing it to be 

 much swollen. 



While still within its albuminous envelope, the embryo moves about 

 inside this envelope by means of cilia on the epidermis, but, upon hatching 

 (Fig. 268), the ciUa disappear and the animal swims by the movement 

 of its tail. The two pairs of external gills become long and branched. 

 For a few days after hatching, the larva spends most of its time clinging 

 to objects in the water by its ventral sucker and lives upon the yolk 



