336 



CLEAVAGE (SEGMENTATION) AND BLASTULATION 



formation of two normal embryos. However, if the first cleavage is at right 

 angles to the plane of bilateral symmetry of the egg, the blastomere containing 

 the gray crescent material will develop a normal embryo, but the other blas- 

 tomere will not do so. 



The above results from isolated blastomeres suggest the following: When 

 the division of the early egg is purely quantitative, so that the resulting blasto- 

 meres contain all of the cytoplasmic substances equally, as in the first one or 

 two cleavage planes in the sea urchin (fig. 166A, B) or the first cleavage in 

 the frog when it bisects the gray crescent, the isolation of the resulting blasto- 

 meres tends to produce complete embryos. Such blastomeres are known as 

 totipotent blastomeres. (See Chap. 8.) However, when cleavage is quali- 

 tative, such as the second cleavage of Amphioxus, the third cleavage of the 

 sea urchin (fig. 166C), or the first cleavage of the frog when it occurs at right 

 angles to the median axis of the embryo, the resulting development depends 

 upon the qualities (that is, ooplasmic substances) resident in the isolated 

 blastomeres. 



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