629 



B 





DEVELOPMENT OF THE FROG 



The formation of polar bodies in the frog is similar to that in 

 Branchiostoma. The first division of the cleavage which follows 

 fertilisation forms two similar cells, each containing, like tho ovum, 

 an upper, black, pigmented portion 

 and a lower, white, yolky portion. 

 The second division is at right angles 

 to the first and forms four similar 

 blastomeres ; the third division is 

 horizontal and separates four small, 

 pigmented, upper blastomeres from 

 four large, yolky, lower blastomeres. 

 By succeeding divisions sixteen and 

 then thirty-two blastomeres arise, 

 after which cleavage becomes 

 irregular, the pigmented cells divid- 

 ing more rapidly than the yolky. 

 The final result is the formation 

 of a blastula (Fig. 487), in which 

 the floor of the blastocoele is com- 

 posed of large yolky cells and the 

 roof of small pigmented cells. At the 

 sides the upper cells merge gradually 

 into the lower. The large cells are the 

 future endoderm, the small cells 

 will give rise to the ectoderm and 

 mesoderm, and both regions difter 

 from the corresponding parts of the 

 blastula of Branchiostoma in being 

 more than one cell deep, though the 

 floor is much thicker than the roof. 

 From this blastula a gastrula is 

 formed, but the process of in- 

 vagination is obscured by the large 

 size and inertia of the yolky cells. 



GASTRULATION 



When gastrulation begins, the 

 two kinds of cells each form half 



Wi\ 



LJ 



V. 



Fig 



467. — btagcs in the- t.ica\ 

 of the frog"s"egg. Of each su -<- 

 two views are given, one show- 

 ing it from the side, the other 

 obhqucly from above. A-l-' 

 show successively stages with 

 two, four, eight, sixteen, thirty- 

 two, and numerous blastomeres. 



