202 HUMAN PLACENTA. 



In all the Rodentia the placenta appears to be situated on the ineso- 

 uietric side of the uterus. 



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

 inembranes 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 allantoic cavity persists (Owen), but 

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

 persistent, and fuses with the non-allantnic part of the subzonal membrane; 

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

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

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

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

 whether the yolk-sack persists till the close of foBtal life, bvit 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-placeutal 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 ( Vespe7'tilio 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 Yespertilio by Owen, The allantoic vessels 

 supply the placenta only. The Cheiroptera are usually imiparous. 



Simiadse and Anthropidae. The foetal 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 iiterine wall, which in the meantime has undergone considerable 

 preparatoi'y 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 gi-ow out, ac- 

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

 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 fi-om 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 diagram- 

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

 separated by a considerable intei'val from the embryo. The yolk-sack is 

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



