380 THE PLACENTA. [BOOK iv. 



centrated towards the region of the decidua serotina. There is 

 evidence that in the formation of the placenta the hypertrophied 

 glandular mucous membrane, having done its work in nourishing by 

 secretory activity the embryo at an early stage, is, at least in its 

 more superficial portions, absorbed, eaten as it were, by the ad- 

 vancing chorionic vascular tufts. This is introductory to the special 

 vascular arrangements of the placenta, the uterine glands making 

 way for the system of blood sinuses ; but even in the full-grown 

 placenta we may recognize, as we have said, that the interchange 

 between mother and foetus is effected not in a wholly mechanical 

 manner by the mere bringing into close juxtaposition the maternal 

 and foetal blood, but also by an activity which we may venture to 

 call secretory on the one hand of the epithelium covering the villi, 

 and on the other hand of the decidual cells, whatever may be the 

 exact origin and nature of each of these kinds of cell. 



As the nutrition of the embryo becomes more and more con- 

 centrated in the altered decidua serotina or placenta, the decidua 

 vera and reflexa, having played their part, are done away with. 

 They are not, however, shed abruptly as in menstruation ; they are 

 returned piecemeal by absorption into the maternal system ; they 

 atrophy until the whole reflexa and the superficial part of the 

 vera is reduced to a mere membrane adherent to the expanded 

 chorion, while the basal portion of the vera remains to grow up 

 after the birth of the foetus into a normal mucous membrane. 



The serotina having become the maternal portion of the pla- 

 centa continues its functions during the whole of the intra-uterine 

 life of the embryo. When the term of the maternal nutrition of 

 the embryo is ended and birth takes place, there is a sudden dis- 

 ruption of tissue along the line of the decidual layer, either where 

 this joins the muscular coat, the whole mucous coat being subse- 

 quently renewed, or at some little distance from it, the ' basal 

 remnants ' of the glands being left to grow up into the new mucous 

 lining; and the transformed serotina, like the changed mucous 

 membrane of menstruation but even more suddenly and abruptly, 

 is shed as the " after-birth." With the placenta there are also shed 

 the so-called * membranes,' that is to say the amniotic membranes 

 together with the membranous remnants of the vera and reflexa, 

 which have become adherent to and fused with these. Hence 

 ultimately the whole decidua, the whole transformed mucous 

 membrane of the pregnant uterus, like the changed mucous 

 membrane of the menstruating uterus is, though in a different 

 manner, cast off. 



We may add that the form and structure of the placenta and 

 the mode of connection between the mother and the embryo 

 differ in different placenta! animals ; in all cases, however, the 

 blood of the chorionic villi of the embryo are bathed in sinus- 

 like blood-spaces of the mother. In all cases too there is a 



