TYPES OF CLEAVAGE 



295 



Fig.' 143. Stages in formation of the blastocoel in the cleaving egg of Rana pipiens 

 taken from stained sections. (A) Eight-cell stage; blastocoel appearing particularly 

 between micromeres. The macromeres form the floor of the developing blastocoel. 

 (B, C) Later stages of formation of the blastocoel. Blastocoel situated at animal pole. 

 Yolk-laden, vegetal pole cells form floor of the blastocoel while smaller, animal pole 

 cells form its sides and roof. (D) Blastocoel at beginning of gastrulation. 



(figs. 141 E; 142F). The fourth set of cleavages, both in Rana sylvatica and 

 Rana pipiens, in eggs that are spawned naturally, are oriented in a meridional 

 direction (figs. 141F; 142F). These furrows first involve only the animal 

 pole micromeres, but later meridionally directed furrows begin to develop in 

 the yolk-laden macromei-es (figs. 141F; 142F). 



The cleavage of the various blastomeres of the egg to this point tends to 

 be synchronous, and is comparable to that of Amphioxus. However, from 

 this time on asynchronism is the rule and different eggs in a given lot manifest 

 various degrees of irregularity. Exceptional eggs may occur in which the next 

 two cleavage planes resemble the fourth and fifth series of planes in Amphioxus. 

 But, on the whole, the micromeres divide faster than do the macromeres and 

 thus give origin to many small, heavily pigmented, animal pole cells, while 

 the macromeres or vegetal pole cells are larger and fewer in number. The 

 smaller pigmented cells creep downward gradually in the direction of the 

 larger vegetal pole cells (figs. 141G-I; 142G-K). The latter migration of the 



