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TEXTBOOK OF ZOOLOGY 



the equator of the egg- and somewhat above it. This produces four 

 cells at the animal pole that are smaller than the four cells at the 

 vegetal pole. Subsequently the cells at the animal pole (micromeres) 

 divide more rapidly than the cells at the vegetal pole, for they con- 

 tain less yolk. Such cleavage is known as unequal, and as divisions 

 proceed the cells at the animal pole become smaller and more numer- 

 ous than those at the other pole. The final result of the following 

 divisions is to form a hollow sphere, the Uastula, whose cavity is 

 known as the hlastocoele. The blastula is essentially one cell layer 

 thick, although in reality some cells have been crowded from the sur- 

 face, giving the appearance of additional layers. 



micromeres 



m&cromcres 



TROGLEGS, CK065 aSCTlOM OF BLASTULA 



Fig. 288. — Section tlirough blastula stage of developing frog. (Courtesy of General 



Biological Supply House.) 



Gastrulation. — Gastrulation begins with the appearance of a 

 small groove slightly below the equator of the egg. The upper 

 edge of this groove is known as the dorsal lip of the hlastopore. 

 Coincident with the appearance of this groove, the pigmented animal 

 cells begin to grow down over the white vegetal, or yolk, cells; and 

 the dorsal lip moves downward as the line advances. It also extends 

 its edges in a crescent shape laterally around the egg until they 

 finally meet to form a circle. The area enclosed by this circle shrinks 

 as the cells grow down from all sides and its rim moves downward. 

 Cells on the side where the groove began advance more rapidly than 



