264 EMBRYOLOGY. 



These are distributed as a network in the septa, as well as in the basal 

 and closing plates of Winkler, and they begin with narrow openings, 

 which connect with the intervillous spaces. At the margin of the 

 placenta they are joined together, and thereby produce the marginal 

 sinus (Plate II. ), or the ring-like sinus of the placenta. This, however, 

 is not to be regarded as a vessel of uniform calibre, but as a system 

 of irregular spaces joined together. 



In virtue of the conditions described, the chorionic villi are directly 

 bathed by the maternal blood. At the same time, from what has 

 already been said, it is to be seen that the motion of the blood is 

 retarded, owing to the great enlargement of the blood-courses, and 

 that it is irregular, corresponding to the form of the intervillous 

 spaces. In general the motion of the blood is from the middle and 

 from the convex side of the placenta, where the arteries chiefly enter, 

 toward its concave surface and its margin. 



The question as to the significance and the origin of the intervillous 

 blood-spaces coTistitntes the key to the comprehension of the structure 

 of the placenta. 



According to one view, which for a long time was the dominant 

 one in Germany, and is defended by Kollikbr, Langhans, and others, 

 the intervillous spaces originally have no connection with the maternal 

 blood-system. Developmentally they are nothing but spaces between 

 chorion and uterine mucosa, and owe their existence to the fact that 

 the two structures have not everywhere come in contact, but have 

 acquired firm connection only by means of the tips of the villi. The 

 spaces in the earliest stage would be bounded by the epithelium of 

 the villi and the maternal mucosa. Langhans therefore designates 

 them as placental spaces. According to this view they would acquire 

 their blood-contents later only, and in this way, as K6lliker ex- 

 presses it : " The proliferating chorionic villi everywhere corrode, 

 and in part destroy the maternal placental tissue, and thus produce 

 an opening of their vessels, which must naturally lead to a gradual 

 penetration of the maternal blood into the intervillous spaces." 



This view has been modified by other observers (Bbaxton Hicks, Ahlpbld, 

 RUGB, and others) to this extent, that the intervillous spaces, even in the 

 mature placenta, do not normally contain blood nor have connection with 

 the maternal blood-vessels. The almost universally received views concerning 

 placental nutrition are thus called in question. The denial of a regulated 

 blood-circulation has induced the further hypothesis, that a uterine milk, 

 as in the Ruminants, is secreted by the cells of the decidua serotina into the 

 intervillous spaces, and is taken op by the foetal villi. 



