204 HUMAN PLACENTA. 



Altliough the vascular supply is cut off from the chorion la?ve, the 

 processes on its surface do not completely abort. It becomes, as the time 

 of birth approaches, more and more closely united with the reflexa, till the 

 union between the two is so close that their exact boundaries cannot be 

 made out. The umbilical vesicle (fig. 151, nb), although it becomes greatly 

 reduced in size and flattened, persists in a recognisable form till the time of 

 birth. 



As the embryo enlarges, the space between the decidua vera and 

 decidua reflexa becomes reduced, and finally the two parts unite together. 

 The decidua vera is mainly characterised by the presence of peculiar 

 roundish cells in its subepithelial tissue, and by the disappearance of a 

 distinct lining of epithelial cells. During the whole of pregnancy it 

 remains highly vascular. The decidua reflexa, on the disappearance of the 

 vessels in the chorion Iseve, becomes non-vascular. Its tissue undergoes 

 changes in the main similar to those of the decidua vera, and as has been 

 already mentioned, it fuses on the one hand with the chorion, and on the 

 other with the decidua vera. The membrane resulting from its fusion 

 with the latter structure becomes thinner and thinner as pregnancy ad- 

 vances, and is reduced to a thin layer at the time of birth. 



The placenta has a somewhat discoidal form, with a slightly convex 

 uterine surface and a concave embryonic surface. At its edge it is con- 

 tinuous both with the decidua reflexa and decidua vera. Near the centre 

 of the embryonic surface is implanted the umbilical cord. As has already 

 been mentioned, the placenta is formed of the decidua serotina and the 

 foetal villi of the chorion frondosum. The fcetal and maternal tissues are 

 far move closely united (fig. 152) than in the forms described above. The 

 villi of the chorion, which were originally comparatively simple, become 

 more and more complicated, and assume an extremely arborescent form. 

 Each of them contains a vein and an artery, which subdivide to enter the 

 complicated ramifications; and are connected together by a rich anastomosis. 

 The villi are formed mainly of connective tissue, but are covered by an epi- 

 thelial layer generally believed to be derived from the subzonal membrane; 

 but, as was first stated by Groodsir, and has since been more fully shewn 

 by Ercolani and Turner, this epithelial layer is really a part of the cellular 

 decidua serotina of the uterine wall, which has become adherent to the villi 

 in the development of the placenta (tig. 161, g). The placenta is divided 

 into a number of lobes, usually called cotyledons, by septa which pass 

 towards the chorion. These septa, which belong to the serotina, lie be- 

 tween the arborescent villi of the chorion. The cotyledons themselves 

 consist of a network of tissue permeated by large vascular spaces, formed 

 by the dilatation of the maternal blood-vessels of the serotina, into which 

 the ramifications of the fo3tal villi project. In these spaces they partly 

 float freely, and partly are attached to delicate trabeculse of the maternal 

 tissue (fig. 161, G). They are, of course, separated from the maternal blood 

 by the uterine epithelial layer before mentioned. The blood is brought to 

 the maternal part of the placenta by spirally coiled arteries, which do not 

 divide into capillaries, but open into the large blood-spaces already spoken 

 of. From these spaces there pass off oblique utero-placental veins, which 

 pierce the serotina, and form a system of large venous siunses in the 

 adjoining uterine wall (fig. 152, F), and eventually fall into the general 

 uterine venous system. At birth the whole placenta, together with the 



