372 



METAZOAN PHYLA 



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 

 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. 



B 



Medu/fary 

 'groove 



H 



Yolk 

 plug 



Pharyngeal s/ifs 



F E 



Nofochorcf 



Neural -f-ube 



Mouth -^MmmBS^ ^ ^ ^ 

 in^agmafion 

 I J Yolk K 



Ficj. 253. — Early stages in the development of a frog. A, egg cell, before cleavage. 

 B, two-cell stage, and C, four-cell. Z), blastula, and E, section of it. F, beginning gas- 

 trulation by epibole, and G, the process more advanced. H, stage showing the yolk plug, 

 and 7, somewhat later. J, stage v-ith pharyngeal arches and slits; K, median section of 

 same stage. From specimens and Ziegler models. 



While still within its albuminous envelope, the embryo moves about 

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

 (Fig. 254), the cilia 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 

 still contained in the archenteron. About the time the yolk is used up 

 a connection appears between the cavity of the stomodeum and that of 

 the anterior end of the archenteron and another between the cavity 

 of the proctodeum and that of the posterior end of the archenteron, 

 which thus becomes converted into an alimentary canal. The animal 



