246 HUMAN PLACENTA. 



Although the vascular supply is cut off from the chorion laeve, 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 round- 

 ish 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 laeve, becomes non-vascular. Its tissue undergoes changes in the 

 main similar to those of the decidua vera, and as has been already mention- 

 ed, 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 struc- 

 ture becomes thinner and thinner as pregnancy advances, 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 continu- 

 ous 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 foetal and maternal tissues are far more 

 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 complicat- 

 ed ramifications ; and are connected together by a rich anastomosis. The 

 villi are formed mainly of connective tissue, but are covered by an epithelial 

 layer generally believed to be derived from the subzonal membrane ; but, as 

 was first stated by Goodsir, 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 (fig. 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 between the 

 arborescent villi of the chorion. The cotyledons themselves consist of a net- 

 work 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 foetal villi project. In these spaces they partly float freely, and partly are 

 attached to delicate trabeculae of the maternal tissue (fig. i6i,). 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 pla- 

 centa by spirally coiled arteries, which do not divide into capillaries, but 



