438 OUTLINES OF CHORDATE DEVELOPMENT 



only simple contact with the uterine wall, nutritive inter- 

 changes between embryo and parent are carried on only with 

 some difficulty. These forms have been termed the achoria 

 or aplacentalia, although strictly speaking these terms are 

 misnomers, for a simple chorion is present and a placental 

 relation does exist, although to a very limited degree. 



All of the Eutherian Mammalia may then be termed choriata 

 or placentalia, and it is characteristic of these forms that upon 

 the outer surface of the chorion there develop elevations or 

 papilla? known as the villi (Figs. 153, 161, 184). 



The villi are the organs of primary importance in effecting 

 the nourishment of the embryo, and the essential characters, 

 as well as many of the minor characters of the placenta, 

 depend upon the form, mode of distribution, and other 

 characteristics of the villi. 



The chief variations in the characteristics of the villi may be 

 enumerated as follows: they may be trophoblastic or tropho- 

 dermic in origin; they may be simple papilla? or complexly 

 branching, dendritic outgrowths; they may develop very early 

 in embryonic history or very late after a considerable period 

 of intra- uterine life; they may be almost non- vascular or very 

 highly vascularized from the embryonic circulation (umbilical 

 arteries and veins); they may be distributed quite uniformly 

 over the greater part of the chorionic surface or definitely 

 grouped and restricted to certain areas; when restricted they 

 may be grouped in definite patches or cotyledons, scattered 

 over the chorion, like polka-dots, or they may be restricted to 

 certain general zones or bands, or to single large circular areas, 

 or arranged in still other ways; they may be in contact with 

 the maternal mucous epithelium lining the uterine cavity, or 

 with its connective tissue stroma, or with the endothelium of 

 the uterine vessels, or actually bathed directly in the maternal 

 blood stream. 



Several classifications of placenta? have been formulated; 

 based upon one or another of these conditions. While none of 

 these represents a natural classification, we may outline certain 

 of the more important, as a convenient way of stating the 



