THE r(ETAL MEMBEANES OF MAN. 267 



second of the hypotheses cited. " Villi and the tissue of the decidua," 

 he says, " become shoved into each other, as one can interlock the 

 outspread fingers of the two hands. If now the blood-vessels of the 

 serotina be followed, one will recognise here the greatly enlarged 

 capillary network of the surface, upon which the egg comes to lie 

 when it lodges. But its innumerable vessels apparently continue 

 with the sprouts of the decidua to grow toward the villi, and 

 become distended and more voluminous ; on the other hand the villi 

 increase rapidly in size, and thus it is intelligible that the new 

 branches of the villi, whose stems have, as it were, sucked themselves 

 fast in the decidua by means of their tips, at once encounter the en- 

 larged capillaries of the surface, and press forward against these and 

 break into them." 



The weightiest objection that can be brought against this inter- 

 pretation is the assertion of many investigators that the chorionic 

 villi are not covered with a mantle of maternal tissue, and that the 

 intervillous spaces are not lined with vascular endothelium. How- 

 ever, it is precisely upon this point that more exhaustive and 

 especially ontological investigations are desirable. For one is not 

 at liberty to draw conclusions from the conditions of "delivered" 

 placentae, since degeneration may have taken place. Moreover 

 Turner and Leopold claim to have demonstrated endothelia at 

 certain places of the intervillous spaces. But especially decisive 

 here appear to me to be, first, the important investigations which 

 Waldeyer has recently published upon the placental circulation in 

 Man, and, secondly, Keibel's very noteworthy preliminary commu- 

 nication upon the embryology of the human placenta. 



Waldeyer has injected the maternal blood-vessels of placentse 

 which still possessed their normal attachment to the uterus, and has 

 prepared sections through the hardened organ. He finds that the 

 intervillous spaces are nothing else than the enormously enlarged 

 maternal blood-vessels, and that at many places there is still present 

 outside the villous epithelium a layer of flat cells, which he is inclined 

 to interpret as vascular endothelium. He appropriately compares 

 the intrusion of the chorionic vilU into the intervillous blood-spaces 

 with the ingrowth of the arachnoideal villi into the blood-sinus of 

 the dura mater, carrying before them invaginations of the endothelial 

 covering of the latter. 



Keibel has investigated by means of sections a well preserved 

 and prepared human embryo, which was in about the middle of the 

 fourth week. He saw the villi (fig. 150 Z), which were proviced 



