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CLEAVAGE (SEGMENTATION) AND BLASTULATION 



Fig. 144. Cleavage in the rabbit egg. (After Gregory, '30.) (A) One-ceil stage. (B) 

 Two primary blastomeres, one larger than the other. (C) Eight-cell stage. (D) Sixteen- 

 cell stage. (E) Morula stage of 32 cells. (F) External view of stage approximating 

 that in (G). (G) Inner cell mass and blastocoeiic cleft showing in embryo, about IV2 

 hours after copulation. (H) inner cell mass and blastocoeiic space in embryo, approxi- 

 mately 90 hours after copulation. Entoderm cells have not yet appeared. 



pigment cells is marked toward the end of the blastular period and during 

 gastrulation. Cf. figs. 141 H-L; 142H-L. 



The blastocoel within the mass of blastomeres of the cleaving egg of the 

 frog forms somewhat differently from that in Amphioxus in that the cavity 

 arises nearer the animal pole. The smaller micromeres of the animal pole, 

 therefore, are more directly involved than the macromeres of the vegetal pole. 

 Beginning at the eight-cell stage, a spatial separation is present between the 

 four micromeres at the animal pole. The floor of this space or beginning 

 blastocoel is occupied by the yolk-laden macromeres (fig. 143 A, B). As 

 development proceeds, this eccentricity of position is maintained, and the 



