ris- UTEROGESTATION. 



as tliese latter, while they still continue to grow to some extent, do not 

 increase in number, or in the complication of their ramifications, they 

 gradually become more sparse, thin, and elongated, and lose their 

 vascularity. 



An active process of increase meanwhile is going on in the villi 

 placed on the uterine side of the ovum, by which, while they become 

 larger and longer, and penetrate more deeply into the uterine decidua, 

 they also become more minutely and extensively ramified. Changes at 

 the same time occur in the disposition of the decidua placentalis, by 

 which it receives into its substance, and is more and more intimately 

 interlocked with the developing chorionic villi. In the earlier stages, 

 as up to the eighth or ninth week, the foetal and maternal structures 

 may be separated by the withdrawal of the villi from the recesses of 

 the decidua in which they are sunk ; but by the middle of the third 

 month, this becomes no longer possible in consequence of the closer 

 combination or interlocking of the two structures ; in the remaining 

 half of the third month the union becomes more intimate, and by 

 the middle of the fourth month the completion of the placenta is 

 etfected by the continued increase in size and modification of the 

 structure of the maternal and foetal elements. 



Structure of the Placenta. — At the time when the placenta has 

 attained its characteristic form and peculiar structure, or after the 

 fourth month of pregnancy, it forms a large discoid or lenticular mass 

 interposed in a limited space between the foetal membranes and the 

 uterus. It presents a fcetal and a uterine surface, the former having 

 implanted into it, usually near the middle, the umbilical cord, which 

 carries to the placenta the umbilical arteries and veins of the foetus, 

 and is covered by a tubular prolongation of the amnion, passing over 

 it from that membrane Avhere it lines the placenta to the abdominal 

 integument of the foetus. The placenta continues to increase in size 

 with the foetus, and when it has attained its full dimensions, it has a 

 width of from seven to eight inches, and a thickness of about one 

 inch and a quarter. But towards the circumference it rapidly thins, 

 where it becomes continuous with the chorion and decidua. The foetal 

 surface is covered by the chorion and amnion, and presents the larger 

 divisions of the umbilical vessels before they dip into the substance. 

 The uterine surface shows a subdivision into a number of large lobes, 

 sometimes called cotyledons, which are covered with a layer of decidua 

 (d. serotina) passing over the whole of this surface, and sending septal 

 prolongations into the placenta between the lobes, which in some places 

 run almost as far as the fcetal surface. 



The more uniform substance of the placenta (parenchyma) within 

 these lobes, consists, on the one hand, of highly-developed and compli- 

 cated tufts of foetal villi, which adhere to the chorion by vascular stems 

 of considerable size and strength, and subdivide again and again into 

 very complex ramifications ; and on the other, of certain dilated vascular 

 spaces continuous with the uterine vessels, the outlines of which follow 

 closely the ramifications of the villi throughout every inflection of their 

 surface. These spaces are doubtless to be regarded as belonging to 

 the maternal system, but their exact nature it is very difficult to deter- 

 mine in the fully formed condition. They probably originally. possess 

 walls of their own, and arc contained in abounding substance of uterine 

 or decidual tissue : but this has become so reduced in thickness, or so 



