MAMMALIA. 249 



the amnion. The area of adhesion between the outer part of the allantois 

 and subzonal membrane gradually spreads over the whole interior of the 

 subzonal membrane, and vascular villi are formed over the whole area of 

 adhesion except at the two extreme poles of the egg. The last part to be 

 covered is the ventral side where the yolk-sack adjoins the subzonal mem- 

 brane. 



During the extension of the allantois its cavity persists, and its inner part 

 covers not only the amnion, but also the yolk-sack. It adheres to the am- 

 nion and supplies it with blood-vessels (Bischoff). 



With the full growth of the allantois there is formed a broad placental 

 zone, with numerous branched villi, fitting into corresponding pits which be- 

 come developed in the uterine walls. The maternal and fcetal structures be- 

 come closely interlocked and highly vascular ; and at birth a large part of 

 the maternal part is carried away with the placenta ; some of it however still 

 remains attached to the muscular wall of the uterus. The villi of the chorion 

 do not fit into uterine glands. The zone of the placenta diminishes greatly 

 in proportion to the chorion as the latter elongates, and at the full time the 

 breadth of the zone is not more than about one-fifth of the whole length of 

 the chorion. 



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

 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 deciduate than in the Dog. 

 In the Grey Seal (Halichcerus gryphus, 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 sur- 

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

 a greyish membrane formed of the coalesced terminations of the fcetal villi. 



The structure of the placenta in Hyrax 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 Carnivora, till the close of fcetal 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 

 zonary 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 



