244 HUMAN PLACENTA. 



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

 and non-placental 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 Ptero- 

 pus by Rolleston, and in Vespertilio by Owen. The allantoic vessels supply 

 the placenta only. The Cheiroptera are usually uniparous. 



Simiadae 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 

 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 forma- 

 tion of this capsule is a matter of inference and not of observation. During 

 the first fortnight of pregnancy villi grow out, according 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 extremely 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 

 derived 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 pro- 

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

 hypoblastic 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 condi- 

 tion of the embryo and of its membranes at this period is shewn diagramma- 

 tically in fig. 147, 5. Around the embryo is seen the amnion, already sepa- 

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

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

 an earlier stage. The allantoic stalk is shewn at al. Both it and the stalk 

 of the yolk-sack are enveloped by the amnion (am}. The chorion with its 

 vascular processes surrounds the whole embryo. 



It may be noted that the condition of the chorion at this stage is very 

 similar to that of the normal diffused type of placenta, described in the 

 sequel. 



While the above changes are taking place in the embryonic membranes, 

 the blastodermic vesicle greatly increases in size, and forms a considerable 

 projection from the upper wall of the uterus. Three regions of the uterine 



