CHAP, ii.] PREGNANCY AND BIRTH. 1121 



arteries, continuations of the primitive aorta, the allantoic 

 arteries, subsequently called umbilical arteries, make their 

 appearance. These carry the blood of the embryo to the villi 

 of the chorion ; from thence it is returned at first to two veins, 

 but ultimately to a single vein running in company with the 

 umbilical arteries, and called the umbilical vein. 



At first all the villi over the whole surface of the chorion 

 except at two opposite poles are thus supplied with blood, 

 but ultimately the supply is restricted to that part of the 

 chorion which is applied to the decidua serotina. Here the 

 villi become developed into large and conspicuous vascular 

 tufts, whereas over the rest of the chorion they soon atrophy ; 

 and here the placenta is formed through a number of complex 

 changes, the details of which have been and still are the subject 

 of much discussion, changes by which the whole region, stretch- 

 ing from the basal portion of the uterine glands, or even from 

 the uterine muscular coat, to the connective tissue which carries 

 the capillary loops in which the umbilical arteries end, is so 

 altered that it becomes difficult to say which are maternal, 

 which are embryonic elements, which structures are of glandu- 

 lar and true epithelial origin, which of connective tissue or 

 epithelioid origin. . 



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 advancing 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 

 that the interchange between mother and foetus is effectea not 

 in a wholly mechanical manner by the mere bringing into 

 close juxtaposition the maternal and fcetal 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 

 concentrated in the altered decidua serotina or placenta, tlu- 

 decidua vera and reflexa, having played their part, are done 

 away with. They are not, however, shed abruptly as in men- 

 struation ; they are returned piecemeal by absorption into tin- 

 maternal system ; they atrophy until the whole reflexa and the 

 superficial part of the vera is reduced to a mere membram- 

 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 



71 



