THE FCETAL MEMBRANES OF MAMMALS. 



237 



maternal blood- passages is full of significance for the formation of 

 the placenta in the Deciduata as opposed to that of the Indeciduata. 



The second form, the discoid placenta, is characteristic of the- 

 Rodentia, the Insectivora, the Chiroptera and Prosimiae, the Apes and 

 Man. Here the portion of the chorion devoted to the formation, of 

 the placenta is small ; but in compensation for this the tufts of villi 

 (fig. 138 V) are very highly developed; the union between placenta, 

 uterina (M) and placenta 

 foetalis (f) is most in- 

 timate ; the maternal 

 blood-spaces (d 1 ), in the 

 case of the Apes and 

 Man at least, are, as no- 

 where else, enormously 

 distended, so that the 

 villi of the chorion (V) 

 appear to sink directly 

 into them and to be 

 bathed immediately by 

 the maternal blood. 



Since we shall occupy 

 ourselves more at length 

 in the next chapter 

 with the human pla- 

 centa, which belongs to 

 this type, these few 

 remarks may suffice for 

 the time being. 



I close this section 

 with a reference to the 

 high systematic signifi- 

 eance of the embryonic 

 accessory organs of Ver- 

 tebrates. They present, 

 as we have seen, such 

 great and striking dif- 

 ferences in the separate 

 classes, that the utilisa- 

 tion of them for systematic purposes which has been made by 

 MILNE-EDWARDS, OWEN, and HUXLEY was natural. 



All lower Vertebrates, Amphioxus, Cyclostomes, Fishes, Dipnoi 



Fig. 138. Diagrammatic representation of the finer struc- 

 ture of the human placenta according to the hypothesis 

 of TURNER. 



F, Foetal, M, maternal placenta; e', epithelium of the 

 ma.ernal placenta; d, foetal, d', maternal blood- 

 vessels ; V, villus ; ds, decidua serotina of the human 

 placenta; t, t, trabeculae of the serotina running to 

 the foetal villi ; ca, convoluted artery which sinks into 

 the blood-space d' ; up, one of the utero-placental veins 

 conveying blood from the latter; x, a continuation 

 over the villus of maternal tissue lying outside the 

 epithelial layer e' which represents either the endo- 

 thelium of the maternal blood-vessels or a delicate 

 connective tissue pertaining to the serotina, or both 

 together. The layer e' consists, at all events, of ma- 

 ternal cells derived from the serotina. The foetal 

 epithelial layer is no longer to be seen on the villi of 

 the completely formed human placenta. 



