THE EARLY DEVELOPMENT OF THE MAMMAL 437 



5. The Placenta 



Among the Eutherian Mammalia placentation may be de- 

 fined as an intimate relation between a portion of the uterine 

 mucosa and a part or the whole of the chorionic membrane 

 (trophoblast) of the blastodermic vesicle. This relation may 

 involve merely the close apposition of these two tissues, or their 

 actual fusion. Further, in order that this relation may be 

 effective in the nutrition of the embryo, which is in fact its 

 whole raison d'etre, the blood vessels of the allantois become 

 closely associated with the related chorionic and maternal 

 tissues. 



All of the structures thus associated in effecting the nutritive, 

 respiratory and excretory interchanges between embryo and 

 maternal organism, may collectively be termed the placenta. 

 It is thus immediately evident that the placenta is a compli- 

 cated structure and one that is extremely variable, including 

 as it does, these several elements, themselves individually 

 variable. 



Many of the essential facts regarding the establishment and 

 the grosser morphological relations of the placenta have been 

 mentioned in other connections. In the section on the im- 

 piantation of the "ovum" we have seen that the earliest step 

 in placentation is to be found in the relation established 

 between the chorionic ectoderm, whether trophoblast or tropho- 

 derm, and the uterine wall (Fig. 175). This is followed con- 

 siderably later by the vascularization of the chorion thus re- 

 lated to maternal tissues, by the blood vessels of the yolk-sac, 

 as in certain Marsupials (Fig. 174), or of the allantois, as in 

 the Ungulates and rabbit (Fig. 176), or by the vessels of the 

 chorionic mesoderm itself, as in man. 



Upon the character and completeness of the relation between 

 chorionic ectoderm and the uterine tissues depends the funda- 

 mental character or type of the placenta developed later. 

 Among most of the Marsupial Mammalia the surface of the 

 chorion retains its smooth surface and is ordinarily not vascu- 

 larized, either by the yolk-sac or allantois, and since it forms 



