THE F(ETAL MEMBRANES OF MAN. 243 



opposite the attachment, and is described as resembling a cicatrix. 

 It is destitute of blood-vessels, whereas these, as well as uteirne 

 glands, are present in the remaining portion of the overgrowing 

 mucous membrane. The blastula lies in this receptacle now, and 

 even into the beginning of the second month, loosely enclosed ; after 

 opening the capsule the blastula can be removed easily and without 

 injury. 



Whereas in other Mammals only that part of the uterine mucous 

 membrane which contributes to the formation of the placenta is cast 

 off, in the case of Man there occurs a much more extensive ecdysis 

 of the most superficial layer, namely, over the whole inner surface of 

 the uterine cavity. Here, too, the part which is cast off is designated 

 as deciduous membrane or decidua, and three regions are distinguish- 

 able (fig. 139) the part which is thrown around the blastula as 

 decidua reflexa (Dr), the part which forms the floor of the depression 

 in which the ovum has established itself as decidua serotina (Pu), and 

 the remaining portion as decidua vera (Dv). 



In the retiexa we become acquainted with a structure which in 

 this complete form occurs only in the case of Man and the Apes, 

 whereas beginnings of such a structure are also found in other 

 groups, as, e.g., in the Carnivores. Since the foetal capsule does not 

 at first completely fill the uterus, there remains between reflexa and 

 vera a space filled with mucus. 



A second and in many respects astonishing result is, that even 

 in very young and small blastodermic vesicles, as all discoveries 

 agree in showing, a well-developed chorion with abundant villi is 

 fcegun. 



The villi are either distributed over the whole surface of the ovum, 

 or, as in REICHERT'S case (fig. 140 A and B), they leave two opposite 

 poles of the blastula free. They attain a length of one millimetre, 

 and in part have the form of simple cylindrical elevations ; in part 

 they already possess lateral branches. At no place have they fused 

 with the decidua. Like the chorion itself, they consist of two layers 

 of a superficial epithelial layer, derived from the serosa, concern- 

 ing which AHLFELD and KOLLMANN have made very definite and 

 reliable statements, and of a layer of embryonic gelatinous tissue, 

 which extends into the axis of the villi and already appears to bear 

 here and there blood-vessels. 



Unfortunately we have learned nothing from investigations of 

 these youngest of all human embryos concerning the structures 

 within the chorion, the remaining foetal membranes and the 



