GENERAL ZOOLOGY 



Blastula cavity 



Gastrocoel 

 Ectoderm 



Endoderm 



Blastopore 



Ectoderm 



Endoderm 



J) Gastrocoel 



Fig. 5.16. Blastulae and gastrulae of amphibians and birds. A, blastula of Triton, a sala- 

 mander. B, blastula of the domestic fowl. C, gastrula of Triton. D, gastrula of the chick 

 (cf. Fig. 5.6B). (A and C, redrawn from O. Hertwig, Lehrbuch der Entwicklungsgeschichte des 

 Menschen und der Wirbelthiere, 1890; B, redrawn from J. T. Patterson, 1910, Journal of Mor- 

 phology, vol. 21.) 



approximately equal, the blastula cavity is located centrally and surrounded 

 by cells of similar size, as in the amphioxus (Fig. 5.14/)). In the amphibians, 

 where the egg is telolecithal and cleavage is total but unequal, the blastula 

 cavity is in the animal hemisphere and has a roof of small cells and a fioor of 

 large cells (Fig. 5.16.4). Further, in the fishes, reptiles, and birds in which 

 the egg is telolecithal and cleavage occurs only in the blastodisk, or small 

 amount of cytoplasm surrounding the nucleus at the animal pole, the blastula 

 cavity lies between the disk of cells and the underlying, undivided yolk mass 

 (Figs. SAB and C and 5.16.6). The blastula cavity in forms like the 

 amphibians and birds is filled with a solution of food material that diffuses 

 into it from the yolk-laden cells or yolk mass of its floor. 



Cell division continues during the blastula stage. In the frog the cells 

 divide parallel to the surface of the blastula so that the blastula cavity be- 

 comes roofed by several layers of cells. An equatorial belt of cells, which is 

 not visibly difTerent from other parts of the animal hemisphere, is known as 

 the germ ring and plays a very important role in later development (Fig. 

 5.1 5C). The cells formed in this region spread toward the vegetal pole. 



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