MAMMALIA. 207 



At the edge of tlie placental zone there is a very small portion of tlie 

 uterine mucous membrane reflected over the non-placental part of the 

 chorion, which forms a small reflexa analogous with the reflexa in Man. 



The Carnivora generally closely resemble the Dog, but in the Cat the 

 whole of the maternal part of the placenta is carried away with the fcetal 

 parts, so that the placenta is more completely decidnate than in the Dog. 

 In the Grey Seal (Halichcerus gryphiis, Turner, No. 219) the general 

 arrangement of the foetal membranes is the same as in the other groups 

 of the Carnivora, but there is a considerable reflexa developed at the edge 

 of the placenta. The fcetal part of the placenta is divided by a series of 

 primary fissures which give off" secondary and tertiary fissures. Into the 

 fissures there pass vascular laminae of the uterine wall. The general sui'- 

 face of the foetal part of the placenta between the fissures is covered by a 

 greyish membrane formed of the coalesced terminations of the foetal villi. 



The structure of the placenta in Hyi-ax is stated by Turner (No. 221) 

 to be very similar to that in the Felidae. The allantoic sack is large, and 

 covers the whole surface of the subzonal membrane. The amnion is also 

 large, but the yolk-sack would seem to disappear at an early stage, instead 

 of persisting, as in the Carnivoi-a, till the close of foetal life. 



The Elephant (Owen, Turner, Chapman) is provided with a zonary 

 deciduate placenta, though a villous patch is present near each pole of the 

 chorion. 



Turner (No. 220) has shewn that in Orycteropus there is present a zonary 

 placenta, which differs however in several particulars from the normal 

 zonarj' placenta of the Carnivora ; and it is even doubtful whether it is 

 truly deciduate. There is a single embryo, which fills up the body of the 

 uterus and also projects into only one of the horns. The placenta forms a 

 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 longitvulinal diameter of the chorion being occupied by the placenta. 

 The chorionic villi are arborescent, and diff'usely 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 foetal 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 characterized 

 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 placentae are divided into two groups : 

 (1) The polycotyledonary placenta, characteristic of the true Ruminantia 

 (Cervidae, Antilopidae, Bovidae, Camelopardalidae) ; (2) the diflfused placenta 

 ibund in the other non-deciduate Mammalia, viz. the Perissodactyla, the 

 Suidae, the Hippopotaiiiidae, the Tylopoda, the Tragulidae, the Sirenia, the 

 Cetacea, Manis amongst the Edentata, and the Lemuridae. The polycoty- 

 ledonary form is the most differentiated ; and is probably a modification of 

 the diffused form. The diffused non-deciduate placenta is very easily 



