THE PLACENTA. 623 



with those coming from the membranes of the foetus (b, b). There 

 is no absolute adhesion between the two sets of vessels, but only 

 an interlacement of their ramified extremities; and, with a little 

 care in manipulation, the foetal portion of the cotyledon may be 

 extricated from the maternal portion, without lacerating either. In 

 consequence, however, of this intricate interlacement of the vessels, 

 transudation of fluids will evidently take place with great readiness, 

 from one system to the other. 



The form of placenta, therefore, met with in these animals, is one 

 in which the bloodvessels of the fcetal chorion are simply entangled 

 with those of the uterine mucous membrane. In the human sub- 

 ject, the structure of the placenta is a little more complicated, 

 though the main principles of its formation are the same as in the 

 above instances. 



From what has been said in the foregoing chapters, it appears 

 that in the human subject, as well as in the lower animals, the 

 placenta is formed partly by the vascular tufts of the chorion, 

 and partly by the thickened mucous membrane of the uterus in 

 which they are entangled. During the third month, those portions 

 of the chorion and decidua which are destined to undergo this 

 transformation become more or less distinctly limited in their form 

 and dimensions; and a thickened vascular mass, partly maternal 

 and partly fcetal in its origin, shows itself at the spot where the 

 placenta is afterward to be developed. This mass is constituted in 

 the following manner. 



It will be recollected that the villi of the chorion, when first 

 formed, penetrate into follicles situated in the substance of the 

 uterine mucous membrane ; and that after they have become vas- 

 cular, they rapidly elongate and are developed into tufted ramifi- 

 cations of bloodvessels, each one of which turns upon itself in a 

 loop at the end of the vi41us. At the same time the uterine follicle, 

 into which the villus has penetrated, enlarges to a similar extent ; 

 sending out branching diverticula, corresponding with the multi- 

 plied ramifications of the villus. In fact, the growth of the follicle 

 and that of the villus go on simultaneously, and keep pace with 

 each other; the latter constantly advancing as the cavity of the 

 former enlarges. 



But it is not only the uterine follicles which increase in size and 

 in complication of structure at this period. The capillary blood- 

 vessels, which lie between them and ramify over their exterior, 

 also become unusually developed. They enlarge and inosculate 

 freely with each other; so that every uterine follicle is soon covered 



