250 



Basic Structure of Vertebrates 



BLASTOCOELE 



Fig. 207. Cleavage of the frog's egg. (A) First cleavage in process. (B) Two cells. 

 (C) Eight cells. (D) Fourth cleavage complete in animal hemisphere but just be- 

 ginning in the four cells at the vegetal pole. (E) Early blastula, median section. 

 (F, G) Successively later stages, lateral view. (D, F, G, after Morgan: "The 

 Development of the Frog's Egg." By permission of The Macmillan Company, 

 publishers. E, after Marshall. Courtesy, Neal and Band: "Chordate Anatomy," 

 Philadelphia, The Blakiston Company.) 



and soon consists of hundreds of small cells forming what is then called 

 the blastoderm lying as a thin plate of cells on the surface of the yolk 

 (Figs. 208, 209). But there is continuity of blastoderm with yolk only 

 around the periphery of the blastoderm. Elsewhere a thin space, the 

 subgerminal cavity, intervenes between blastoderm and yolk (Fig. 

 209). Comparing this embryo with the blastula stages of Amphioxus 

 and frog, it seems reasonable to interpret it as a blastula whose blasto- 

 coele is the subgerminal cavity, while its blastoderm is the animal 

 region and the yolk-mass is the vegetal region of the embryo. This 

 recognition of a blastula stage, comparable to that of Amphioxus, in 

 the development of a reptile or bird would hardly have been possible 

 but for the intermediate condition exhibited by the amphibian with 

 its moderate yolk-mass and total cleavage. 



The blastula is an essentially one-layer stage of the embryo, the 

 "layer" being the wall of the blastula, whether one cell thick or more 

 than one cell thick. This stage has twofold significance. Its immediate 



