XI.] METADISCOIDAL PLACENTA. 355 



Man. The early stages in the development of the foetal 

 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 infer- 

 ence and not of observation. During the first fortnight of preg- 

 nancy villi grow out, over the whole surface of the ovum. The 

 further history of the early stages is extremely obscure : what 

 is known with reference to it will be found on p. 335 et seq. ; we 

 will 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 niesoblast 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, though 

 not forming a continuous tube. The blood-vessels of the 

 chorion are derived from the usual allantoic arteries and vein. 

 The general condition of the embryo and of its membranes at 

 this period is shewn diagrammatically in Fig. 114, 5. Around 

 the embryo is seen the amnion, already separated by a consider- 

 able interval from the embryo. The yolk-sac is shewn at ds. 

 Eelatively 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-sac 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 pla- 

 centa, 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 wall, in relation to the 



232 



