DEVELOPMENT OF BIMANA. T49 



vera,' or ' d. uteri.' Long and large villi extend from the chorion 

 into the decidua, and at this period (latter half of the first month) 

 there may be traced, upon its inner surface, orifices of canals that 

 lead into the uterine sinuses. The maternal blood already flows 

 freely into the maternal chamber, and, after passing everywhere 

 among the villi, is returned into the uterine veins. Thus a tempo- 

 rary placenta is formed analogous to the diffused form described 

 in Cetacea and certain Unyuluta. But soon the villi increase in 

 length and size on the side of the chorion next the uterine wall, 

 and decrease on the opposite side, which becomes smooth or bald ; 

 this, pressing upon the hydroperionic space, finally obliterates it, 

 and arrests the flow of blood to that part of the circumference of 

 the chorion. On the other part, next the uterine wall, a circular 

 space is left, like a meniscus, round the circumference of which 

 decidual growths pass from the uterus to attach themselves to the 

 chorion, and form the margin of the true placenta ; then, as the 

 uterus enlarges, concomitantly with the expansion of the ovum, 

 a decidua, called ' serotina,' is reproduced to form the basis of the 

 maternal placenta, from which septal processes extend grouping 

 the developed villi of the chorion, or foetal placenta, into lobes. 

 With the further growth of the placenta these lobes become usually 

 more and more confluent, the foetal also becomes more blended 

 with the maternal part, until a structure results, as exposed 

 in the section of the placenta and placenta] area of the uterus, 

 fig. 582. 



The line, u, u, indicates the extent of the uterine wall ; ud is 

 f decidua serotina;' dp deciduous septa, p placenta, ch chorion, 

 am amnion, vf foetal blood-vessels, v, v villi, us uterine venous 

 sinuses, a, a uterine ( curling arteries.' The two foetal arteries 

 (allantoic or umbilical) communicate by a cross branch near the 

 placenta! end of the funis, beyond which they spread in large 

 branches over a considerable part of the free surface of the 

 placenta, and subdivide dichotomously in the chorion, two or 

 three times, before they penetrate the placenta! substance to 

 ramify in the villous processes called ' placenta! tufts.' The 

 stems of these are rooted in the chorion, and are tough and 



o 



fibrous. Each tuft consists of an outer coriaceous and an inner 

 soft tissue : a distinction which is continued to the terminal villi, 

 fig. 583, as shown in the end of one from a stale placenta in 

 which the inner vascular substance had shrunk away from the 

 outer epithelial sheath, ib. b. From the third to the sixth 

 month the arteries of the villi terminate in a rich capillarv plexus 

 at their periphery, ib. a. The veins from the capillaries unite 



