340 COMPARATIVE ANATOMY 



becomes to a greater or less extent torn away at birth (decidua), 

 the placenta being therefore spoken of as deciduate. In these 

 cases, the placental part of the chorion does not extend all round 

 the embryo. In the zonary placenta only the two opposite poles 

 of the chorion are .more or less free from vascular villi, and this 

 girdle-like form occurs in the Carnivora, as well as in the Elephant 

 Hyrax, and Orycteropus. In Lemurs and Sloths, the placenta is 

 dome- or bell-shaped, while in Myrmecophaga, Dasypodidae (Arma- 

 dilloes), and Primates (Fig. 9) it forms a discoidal mass on the 

 dorsal side of the embryo (metadiscoidal form). The discoidal 

 placenta of Rodentia, Insectivora, and Cheiroptera has probably 

 not arisen, like that just mentioned, from a diffused type, but was 

 originally restricted to a discoidal area, owing to the umbilical 

 vesicle occupying a large surface of the chorion. 



From the above description it is evident that the differences 

 in the form of the placenta are mainly those of degree, and that 

 the latter gives little indication of the systematic position of the 

 animal in question. 



The histological structure of the placenta and the various modifications 

 seen in the maternal mucous membrane cannot be described here ; it is, 

 however, important to note that there is no direct communication between the 

 maternal and foetal blood, and that the maternal capillaries usually enlarge 

 to form sinuses, the walls of which become invaginated by the villi : 

 thus the latter are covered by an epithelium furnished by the maternal 

 tissues (Fig. 272). 



In the course of development the embryo becomes more and 

 more folded off from the yolk-sac (Fig. 8), the stalk of which latter 

 and that of the allantois, enveloped by the base of the amnion, 

 together form the umbilical cord. At birth, the foetal membranes 

 are shed, the intra-abdorninal portion of the allantois persisting as 

 the urachus (comp. p. 358). 



