PART II. THE FORMATION OF THE BLASTOCYST 



THE EARLY UNILAMINAR BLASTOCYST 



a. The just completed blastocyst 



1) Formation of the blastocyst. In the preceding section it 

 was seen that the formation of the blastocyst was plainly fore- 

 shadowed in the 8- to 16-celied stages. Early in cleavage the 

 cells migrate toward the albumen layer, to which they soon 

 become closely appressed. The stage at which the cells arrive 

 and begin to flatten out beneath the zona varies in different 

 eggs. The process may begin in the 5-, 6- or 7-celled stage 

 (fig. 19) or it may be delayed beyond the 16-celled stage. In 

 one 31-celled egg (text fig. 5 B), for example, the blastomeres of 

 only one hemisphere have flattened out and completed this half 

 of the blastocyst; the cells of the other half are rounded and 

 still have gaps between them. In most cases of 15- to 18-celled 

 eggs, however, the blastomeres have definitely established a 

 contact with the albumen and this relation they do not change 

 until, in the large bilaminar blastocysts, the albumen is all 

 absorbed. Hence in blastocysts of the opossum there is no 

 'peritivelline space' nor are the cells at all normally rounded 

 off peripherally in the manner figured by Selenka. The early 

 blastocysts described by him are no doubt damaged specimens 

 in which the cells have shrunken away from the albumen layer. 



Since, therefore, the blastomeres of the 16-celled egg are already 

 arranged in a hollow sphere, the blastocyst is completed at a 

 much earlier stage than, that of Dasynrus. Of nearly four dozen 

 specimens of young blastocysts, each approximating 50 cells, all 

 have completed walls except five (text fig. 5 N). The blastocyst 

 wall is indeed complete in sortie specimens containing from 

 30 to 40 cells. 



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