240 COMPARATIVE HISTORY OF FCETAL MEMBRANES. 



Marsupialia. Our knowledge of the foetal membranes of 

 the Marsupialia is almost entirely due to Owen. In Macropus 

 major he found that birth took place thirty-eight days after 

 impregnation. A foetus at the twentieth day of gestation 

 measured eight lines from the mouth to the root of the tail. 

 The foetus was enveloped in a large subzonal membrane, with 

 folds fitting into uterine furrows, but not adhering to the uterus, 

 and without villi. The embryo was enveloped in an amnion 

 reflected over the stalk of the yolk-sack, which was attached by 

 a filamentary pedicle to near the end of the ileum. The yolk- 

 sack was large and vascular, and was connected with the foetal 

 vascular system by a vitelline artery and two veins. The yolk- 

 sack was partially adherent, especially at one part, to the 

 subzonal membrane. No allantois was observed. In a some- 

 what older foetus of ten lines in length there was a small allantois 

 supplied by two allantoic arteries and one vein. The allantois 

 was quite free and not attached to the subzonal membrane. The 

 yolk-sack was more closely attached to the subzonal membrane 

 than in the younger embryo 1 . 



All Mammalia, other than the Monotremata and Marsupialia, 

 have a true allantoic placenta. The placenta presents a great 

 variety of forms, and it will perhaps be most convenient first to 

 treat these varieties in succession, and then to give a general 

 exposition of their mutual affinities 2 . 



Amongst the existing Mammals provided with a true placenta, the 

 most primitive type is probably retained by those forms in which the 

 placental part of the chorion is confined to a comparatively restricted area 

 on the dorsal side of the embryo ; while the false chorion is formed by the 



"inches in length." On the 12th of August, 1864, "a female Echidna hystrix was 

 " captured .... having a young one with its head buried in a mammary or marsupial 

 " fossa. This young one was naked, of a bright red colour, and one inch two lines in 

 "length." 



1 Owen quotes in the Anatomy of Vertebrates, Vol. ill. p. 721, a description from 

 Rengger of the development of Didelphis azarse, which would seem to imply that a 

 vascular adhesion arises between the uterine walls and the subzonal membrane, but 

 the description is too vague to be of any value in determining the nature of the foetal 

 membranes. 



2 Numerous contributions to our knowledge of the various types of placenta have 

 been made during the last few years, amongst which those of Turner and Ercolani 

 may be singled out, both from the variety of forms -with which they deal, and the 

 important light they have thrown on the structure of the placenta. 



