250 PLACENTA OF THE UNGULATA. 



broad median zone, leaving the two poles free. The breadth of the zone is 

 considerably greater than is usual in Carnivora, one-half or more of the 

 whole longitudinal diameter of the chorion being occupied by the placenta. 

 The chorionic villi are arborescent, and diffusely scattered, and though the 

 maternal and foetal parts are closely interwoven, it has not been ascer- 

 tained whether the adhesion between them is sufficient to cause the ma- 

 ternal subepithelial tissue to be carried away with the fcetal part of the 

 placenta at birth. The allantois is adherent to the whole chorion, the non- 

 placental parts of which are vascular. In the umbilical cord a remnant of 

 the allantoic vesicle was present in the embryos observed by Turner, but in 

 the absence of a large allantoic cavity the Cape Ant-eater differs greatly 

 from the Carnivora. The amnion and allantois were in contact, but no 

 yolk sack was observed. 



Non-deciduate placenta. The remaining Mammalia are charac- 

 terized by a non-deciduate placenta ; or at least by a placenta in which only 

 parts of the maternal epithelium and no vascular maternal structures are 

 carried away at parturition. The non-deciduate placentas are divided into 

 two groups : (i) The polycotyledonary placenta, characteristic of the true 

 Ruminantia (Cervidae, Antilopidae, Bovidae, Camelopardalidae) ; (2) the 

 diffused placenta found in the other non-deciduate Mammalia, viz. the 

 Perissodactyla, the Suidae, the Hippopotamidae, the Tylopoda, the Tragulidae, 

 the Sirenia, the Cetacea, Manis amongst the Edentata, and the Lemuridas. 

 The polycotyledonary form is the most differentiated ; and is probably a 

 modification of the diffused form. The diffused non-deciduate placenta is 

 very easily derived from the primitive type (p. 240) by an extension of the 

 allantoic portion of the chorion ; and the exclusion of the yolk-sack from any 

 participation in forming the chorion. 



The possession in common of a diffused type of placenta is by no 

 means to be regarded as a necessary proof of affinity between two groups, 

 and there are often, even amongst animals possessing a diffused form of 

 placenta, considerable differences in the general arrangement of the em- 

 bryonic membranes. 



Ungulata. Although the Ungulata include forms with both coty- 

 ledonary and diffused placentae, the general arrangement of the embryonic 

 membranes is so similar throughout the group, that it will be convenient to 

 commence with a description of them, which will fairly apply both to the 

 Ruminantia and to the other forms. 



The blastodermic vesicle during the early stages of development lies 

 freely in the uterus ; and no non-vascular villi, similar to those of the 

 Dog or the Rabbit, are formed before the appearance of the allantois. 

 The blastodermic vesicle has at first the usual spherical form, but it grows 

 out at an early period, and with prodigious rapidity, into two immensely 

 long horns ; which in cases where there is only one embryo are eventually 

 prolonged for the whole length of the two horns of the uterus. The 

 embryonic area is formed in the usual way, and its long axis is placed at 

 right angles to that of the vesicle. On the formation of an amnion there 



