CHAP. XLIII.] CHORION. 613 



which the maternal capillary vessels are spread out ; while the vascular loops 

 of the human foetus, as was shown by Professor Weber, dip into the dilated 

 vessels of the mother, which become large venous sinuses, and are thus com- 

 pletely bathed on all sides by the mother's blood. 



The villi increase very much in number and complexity in that 

 part of the chorion which is to become the placenta; while on 

 other parts of the surface they retain the same characters as at a 

 very early stage. Each villus contains a vascular loop, which is 

 directly continuous with the umbilical vessels of the foetus ; and 

 the whole of the blood of the foetus is made to pass through the 

 vessels in the tufts by the forces of the foetal circulation. The 

 cells of which the villi were entirely composed, at a very early 

 period diminish in number; but still several remain towards the 

 apex. 



During this time, the soft membrana decidua has been increas- 

 ing in thickness and vascularity. Its capillary vessels become 

 enormously increased in diameter, and ultimately form small 

 pouches or sinuses containing blood. The foetal tufts come into 

 close relation with the walls of these sinuses, but are still separated 

 by a thin layer of the cellular decidua, and project into their inte- 

 rior, being of course invested with the wall of the sinus, just as 

 the viscera are covered with peritoneum. Such is the relation of 

 the blood-vessels of the foetal placenta to those of the mother, 

 according to the observations of Dr. J. Reid, Weber, and Goodsir. 

 The structures, therefore, which intervene F W- 287. 



between the blood of the foetus and that 

 of the mother, are the following : the 

 walls of the foetal capillaries ; the cells at 

 the extremity of the foetal tufts ; the deli- 

 cate investing membrane covering these; 

 a thin stratum of fluid separating the 

 maternal and foetal portions of the pla- 

 centa, and containing not only the ma- 

 terials for absorption but any substances 

 to be removed from the foetal blood ; the 

 cells of the membrana decidua; and, 

 lastly, the wall of the venous sinus, into 

 which the foetal tuft projects. 



,. ... ,, .,, Extremity of a vi'.lus, showing ca- 



The Cells Upon the Surface Of the VlllUS pillary vessels. After Weber. 



form little groups, and appear to radiate, as it were, from the 

 centre of each collection. This central point, Professor Goodsir 

 regards in the light of a germinal spot or nutritive centre, which 



