202 HUMAN PLACENTA. 



In all the Rodeiitia the placenta appears to be situated on the meso- 

 inetric side of the uterus. 



Insectivora. In the Mole (Talpa) and the Shrew (Sorex), the foetal 

 membranes are in the main similar to those in the rabbit, and a deciduate 

 discoidal placenta is always present. It may be situated anywhere in the 

 circumference of the uterine tube. The allautoic cavity persists (Owen), but 

 the allautois only covers the placental area of the chorion. The yolk-sack is 

 persistent, and fuses with the non-allant'-ic part of the subzonal membrane; 

 which is rendered vascular by its blood-vessels. There would seem to be 

 (Owen) a small decidua renexa. A similar arrangement is found in the 

 Hedgehog (Erinaceus Europeans) (Rolleston), in which the placenta occupies 

 the typical dorsal position. It is not clear from Rolleston's description 

 whether the yolk-sack persists till the close of foetal life, but it seems 

 probable that it does so. There is a considerable reflexa which does not, 

 however, cover the whole chorion. In the Tenrec (Centetes) the yolk-sack 

 and non-placenta] part of the chorion are described by Rolleston as being 

 absent, but it seems not impossible that this may have been owing to the 

 bad state of preservation of the specimen. The amnion is large. In the 

 Cheiroptera ( Vespertilio and Pteropus), the yolk-sack is large, and coalesces 

 with part of the chorion. The large yolk sack has been observed in Pte- 

 ropus by Rolleston, and in Vespertilio by Owen. The allantoic vessels 

 supply the placenta only. The Cheiroptera are usually uniparous. 



Simiadse and Anthropidae. The fcetal membranes of Apes and Man, 

 though in their origin unlike those of the Rodentia and Insectivora, are 

 in their ultimate form similar to them, and may be conveniently dealt 

 with here. The early stages in the development of these membranes in 

 the human embryo have not been satisfactorily observed ; but it is known 

 that the ovum, shortly after its entrance into the uterus, becomes attached 

 to the uterine wall, which in the meantime has undergone considerable 

 preparatory changes. A fold of the uterine wall appears to grow round 

 the blastodermic vesicle, and to form a complete capsule for it, but the 

 exact mode of formation of this capsule is a matter of inference and not of 

 observation. During the first fortnight of pregnancy villi grow out, ac- 

 cording to Allen Thomson over its whole surface, but according to Reichert 



O O 



in a ring-like fashion round the edge of the somewhat flattened ovum, and 

 attach it to the uterus. The further history of the early stages is ex- 

 tremely obscure, and to a large extent a matter of speculation : what is 

 known with reference to it will be found in a special section, but I shall 

 here take up the history at about the fourth week. 



At this stage a complete chorion has become formed, and is probably de- 

 rived from a growth of the mesoblast of the allantois (unaccompanied by the 

 hypoblast) round the whole inner surface of the subzonal membrane. From 

 the whole surface of the chorion there project branched vascular processes, 

 covered by an epithelium. The allantois is without a cavity, but a hypo- 

 blastic epithelium is present in the allantoic stalk, through which it does 

 not, however, form a continuous tube. The blood-vessels of the- chorion 

 are derived from the usual allantoic arteries and vein. The general con- 

 dition of the embryo and of its membranes at this period is shewn cliagram- 

 matically in fig. 147, 5. Around the embryo is seen the amnion, already 

 separated by a considerable interval from the embryo. The yolk-sack is 

 shewn at Js. Relatively to the other parts it is considerably smaller than 



