292 Vertebrate Embryology 



supposes it to be about a week. On entering 

 the uterus the blastodermic vesicle is, in many 

 mammals, covered with a gelatinous secretion 

 from the uterine glands ; this envelope is called 

 titot prorckorwn and must not be confused with 

 the true chorion to be described later. 



The region of attachment of the inner cell 

 mass to the subzonal layer is the position of 

 the future embryo. As development proceeds 

 the inner cell mass spreads out until it forms a 

 layer of cells entirely around the vesicle, inside 

 of the subzonal layer. This growth of the inner 

 mass takes place differently in different animals. 

 The vesicle increases rapidly in size by the 

 flattening and multiplication of the cells, and 

 becomes filled with a fluid of uncertain com- 

 position, probably derived from the wall of the 

 uterus. 



The shape of the enlarged vesicle and the 

 size to which it grows vary in different animals. 

 There is also much variation in different mam- 

 mals in regard to the growth of the subzonal 

 or ectodermal layer, which, in some cases, be- 

 comes thickened, either in a restricted area, or 

 over the entire vesicle, to form what is called 

 the trophoblast. This trophoblast, according 

 to Minot, has two functions, both of which are 



