440 



COMPARATIVE ANATOMY 



Insectivora, and Cheiroptera, though the placenta is also discoidal, 

 it has probably not arisen, like that just mentioned (metadiscoidal 

 form) from a diffuse type, but was originally restricted to a discoidal 

 area, owing to the umbilical vesicle occupying a large surface of the 

 chorion. 1 



From the above description it is evident that the differences 

 in the form of the placenta are mainly those of degree, and that 



Ch crion $ 'efa e sse 



(jhff none/) ithe'L 



T~.ndotk.fi! . 



FIG. 334. DIAGRAM TO ILLUSTRATE THE RELATIONS OF THE FCETAL AND 



MATERNAL VESSELS IN THE HUMAN PLACENTA, SHOWING CHORIONIC AND 



MATERNAL VESSELS (Gefcisse) AND CAPILLARIES, VILLI (Zotten), AND 

 DECIDUA. (After Keibel.) 



the placenta gives little indication of the systematic position of the 

 animal in question. 



In the course of development the embryo becomes more and 

 more folded off from the yolk-sac (Fig. 8), the stalk of which 

 and that of the allantois, enveloped by the base of the amnion, 

 together form the umbilical cord. At birth, the foetal membranes 

 are shed, the intra-abdominal portion of the allantois persisting as 

 a cord, the lira elms, 



1 The histological structure of the placenta and the various modifications 

 seen in the maternal mucous membrane cannot be described here ; it is, how- 

 ever, important to remember that there is no direct communication between the 

 maternal and fretal blood, and that the maternal capillaries usually enlarge 

 to form sinuses, the walls of which become invaginated by the villi ; thus 

 the latter are covered by an epithelium furnished by the maternal tissues (Fig. 

 334). In some cases (c.j/. Mole) part of the maternal tissue is absorbed in the 

 uterus, 



