290 Vertebrate Embryology 



THE BLASTODERMIC VESICLE 



Segmentation has never been observed in 

 man, but the process, we may imagine, is more 

 or less similar to what has been studied in some 

 of the lower mammalia. 



The first cleavage, preceded, of course, as 

 are all of the succeeding cleavages, by a mit- 

 otic division of the nucleus, divides the egg 

 into two equal blastomeres. Following the two- 

 cell stage is the four-cell condition (Fig. 87), 

 although a three-cell stage has been described in 

 some mammals, formed, possibly, by a division 

 of one of the first two blastomeres before the 

 other. 



After the four-cell stage segmentation pro- 

 ceeds with some irregularity, but it is soon 

 evident that the blastomeres are arranging them- 

 selves into an outer layer, close under the zona 

 radiata, and an inner group. The outer cells 

 are somewhat flattened and are known as the 

 subzonal layer; the inner group of cells, sur- 

 rounded by the subzonal cells, is the inner cell 

 mass (Fig. 88). 



A space now makes its appearance between 

 the inner cell mass and the subzonal layer, but 

 it does not separate these two groups of cells 

 on all sides, the inner cell mass remaining at- 



