MAMMALIA. 199 



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. Tlje embryo was en- 

 veloped 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 mem- 

 brane. No allantois was observed. In a somewhat 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 \ 



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 afiinities^ 



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 

 vascular yolk sack fusing with the remainder of the subzonal membrane. 

 In all the existing forms with this arrangement of fcutal membranes, the 

 placenta is deciduate. This, however, was probably not the case in more 

 primitive forms from which these are descended^ The placenta would 

 appear from Ercolani's description to be simpler in the mole (Talpa) than 

 in other species. The Insectivora, Cheiroptera, and Rodentia are the 

 groups with this type of placenta ; and since the rabbit, amongst the latter, 

 has been more fully worked out than other species, we may take it first. 



The Rabbit. In the pregnant female Rabbit several ova are generally 

 found in each horn of the uterus. The general condition of the egg-mem- 

 braues at the time of their full development is shewn in fig. 148. 



The embryo is surrounded by the amnion, which is comparatively small. 

 The yolk-sack {ds) is lai'ge and attached to the embryo by a long stalk. 

 It has the form of a flattened sack closely applied to about two-thirds of the 

 surface of the subzonal membrane. The outer wall of this sack, adjoining 



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

 Eengger of the development of Didelphis azarae, 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. 



^ Vide Ercolani, No. 197, and Harting, No. 201, and also Von Baer, Entwick- 

 lungsgeschichte table on p. 225, part i., where the importance of the limited area of 

 attachment of the allantois as compared with the yolk-sack is distinctly recognised. 



