BLASTULA, GASTRULA, AND GERM LAYERS 331 



the blastocoel or segmentation cavity, is large and nearly central, 

 though not quite, for nearly always the cells at the vegetal pole 

 are larger than those at the animal pole, and the wall conse- 

 quently thicker in the former region. This form of blast ula is 

 commonly regarded as primitive, though hardly typical of the 

 Chordates in general, for among these it is found only in 

 Amphioxus. 



C 



I) 



en 



FIG. 150. Types of Chordate blastulse. A. Amphioxus (coeloblastula). 

 B. Petromyzon. After von Kupffer. C. Noturus (Teleost) (discoblastula). 

 D. Triton (Urodele). After Greil. a, animal pole; c, blastocoel; p, periblast; 

 v, vegetal pole. 



More usually the Chordate ovum contains a considerable 

 amount of yolk, as in the Ganoids and Amphibia. And here 

 the cleavage, though nearly or quite complete in most cases, is 

 decidedly unequal and the form of the blastula is considerably 

 modified in consequence. Here the wall of the blastula is 

 several or many cells thick; the cells of the animal pole are 

 quite small, while the yolk-containing cells, of the vegetal 

 hemisphere, are very large. The blastocoel is therefore quite 

 eccentric, displaced toward the animal pole, and usually 

 much reduced in size (Fig. 150, B, D). 



