378 



METAZOAN PHYLA 



of the egg, called the animal pole, is dark in color, and the lower, or 

 vegetal pole, is light because of the massing of the yolk in that portion. 

 The eggs are holoblastic but, owing to the amount of yolk, undergo 

 unequal cleavage (Fig. 267). The upper and smaller cleavage cells 

 are known as micromeres, the lower and larger ones are the macromeres. 

 A blastula cavity, or blastocoel, is formed, and gastrulation takes place 

 by epibole, a fold of micromeres growing around and inclosing the 

 macromeres, leaving the yolk visible only through the blastopore. This 



Medu/fary 

 'groove 



H 



Yolk 

 plug 



G F 



Pharyngeal s/ifs 



Nofochorof 



, Neural lube 



Archenferon 



Mouth 

 in vagi not/ on 

 I J Yolk K 



Fig. 267. — Early stages in the development of a frog. A, egg cell, before cleavage. 

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

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

 and /, somewhat later. /, stage with pharyngeal arches and slits; K, median section of 

 same stage. From specimens and Ziegler models. 



visible portion of the yolk is termed the yolk plug. Soon after gastrula- 

 tion there is developed a groove, called the medullary groove, running 

 dorsally from the blastopore forward toward what will become the 

 anterior end of the larva. The blastopore gradually becomes obhterated 

 by the contraction of its margins. The embryo now becomes elongated 

 and the head and tail become free. 



Later, and after the embryo has become better developed, a swelHng 

 appears on each side near the anterior end of the body. Below each 

 swelhng is developing a gill arch, and in front of it a depression which 

 moves toward the ventral side of the body and unites with that of the 

 other side to form a ventral sucker. Above the ventral sucker an invagina- 



