THE EARLY DEVELOPMENT OF THE MAMMAL 423 



It is evident from this description that the trophoderm or 

 Syncytiotrophoderm, forms the boundary between the embry- 

 onic and the maternal tissues, and not only effects the implanta- 

 tion of the "ovum," but at the same time establishes the 



FIG. 175. Early stages in the implantation of the blastodermic vesicle of the 

 spermophile (Spermophilus citillus). After Rejsek. A. Unattached vesicle. B. 

 Syncytiotrophoderm just penetrating the epithelium of the uterine mucous 

 membrane. C. Syncytiotrophoderm extending along the basement membrane 

 of the uterine epithelium. D. More highly magnified view of the Syncytio- 

 trophoderm after it has penetrated the tasement membrane and entered the 

 connective tissue of the uterine mucosa. b, Basement membrane of uterine 

 epithelium; ct, connective tissue of the uterine mucosa; e, epithelium of the 

 uterine mucosa; en, endoderm; i, inner cell mass; s, Syncytiotrophoderm; 

 t, trophoblast. 



primary element in the placenta. The trophoderm later 

 becomes vascularized from the mesoderm of the chorion or 

 allantois (yolk-sac in some cases), and acts as the chief absorp- 



