266 EMBRYOLOGY. 



the villous tufts (.F) have grown, and with which the most intimate 

 contact everywhere prevails. Outside the epithelium TURNER de- 

 scribes in addition a thin membrane (a:), which he interprets as an 

 exceedingly thin connective-tissue layer, upon which is probably to 

 be found an endothelial covering which lines the blood-spaces. The 

 cords indicated by t are connective-tissue strands of the maternal 

 mucosa, which join the tips of certain foetal villi with the septa 

 placentae (ds), by which the origin of the so-called attachment- 

 roots (Haftwurzeln) is explained. The great blood-spaces d 1 are 

 simply enormously enlarged, superficially located capillaries of the 

 mucosa. 



The exact determination of the true state of affairs is coupled with 

 great difficulties. 



However, it seems to me that the second of the two hypotheses 

 cited, according to which the intervillous spaces are the enlarged 

 maternal capillaries; is the more probable because the more natural, 

 and the following facts especially appear to me to favor it : 



(1) From a comparative-anatomical point of view it can be main- 

 tained that in all Mammals where a special adaptation to intra-uterine 

 nutrition is developed, the epithelial surfaces of the chorion and the 

 mucous membrane of the uterus lie directly on each other, and with 

 the increase of surface produced by the formation of folds effect 

 mutual ingrowth. An intra-placental fissure, such as LANGHANS 

 and KOLLIKER assume for Man, is found nowhere else among 

 Mammals. We also see in some instances how the capillaries of 

 the uterine mucosa become enlarged and acquire attenuated walls 

 (Rodents, Carnivora, etc.), so that the foetal villi are almost directly 

 bathed in maternal blood. The enlargement of the blood-courses in 

 Man may therefore be regarded as a further elaboration of an already 

 existing arrangement. 



(2) That capillaries become metamorphosed into a cavernous 

 system is also realised in other parts of the human body (corpora 

 cavernosa of the sexual organs), whereas the employment of spaces 

 lying outside the blood-courses as component parts of the, vascular 

 system ivoidd be a phenomenon without analogy. 



(3) In the placenta uterina the capillaries originally present are 

 wanting between the arteries and veins, whereas they ought to be 

 demonstrable, if they have not been converted into the intervillous 

 spaces. 



(4) The exposition which LEOPOLD has given of the development 

 of the placenta in the second month of pregnancy favors the 



