THE UMBILICAL COED AND THE CIIORION. 603 



concerned in the formation of the embryo. It is usual to exclude 

 the amnion from the definition ; but in the case of the human 

 embryo the outer layer of the amnion, or ' false amnion ' as it is 

 commonly called in other Vertebrates, is so directly continuous 

 with the wall of the vesicle that it is better to include it under 

 the same name. 



Thus in Eeichert's ovum (Fig. 1 74) the chorioii is the whole 

 wall of the vesicle, except the embryonic area, a. In His' 

 embryo E (Fig. 188), the chorion forms the entire wall of the 

 vesicle, the embryo being now depressed within its cavity. 



It is the chorion which comes in contact with the walls of 

 the uterus (Figs. 254, 255), and it is from the chorion that the 

 foetal part of the placenta is developed. 



The human chorion is remarkable for its very early and 

 complete separation from the yolk-sac (Figs. 186 to 188); and 

 also for the very early period at which villi are developed from 

 its outer surface. 



Structurally, the chorion consists of an outer layer of epiblast, 

 which from the first is two cells thick ; and an inner and thicker 

 layer of mesoblast, which very early becomes vascular, the blood- 

 vessels being derived from the allantoic arteries and veins, 

 which reach the chorion along the allantoic stalk, and which 

 are, of course, directly continuous with the blood-vessels of the 

 embryo. 



In Keichert's ovum (Figs. 172, 173, 174), the villi are con- 

 fined to a broad marginal zone round the equator, the centres of 

 the two flattened surfaces forming bare patches. At a very 

 slightly later period, in His' embryo E, and in others of about the 

 thirteenth day (Figs. 175, 188), the villi cover the entire surface 

 of the chorion. 



The chorionic villi consist at first entirely of epiblast. They 

 arise as solid buds of epiblast, which become hollow as they in- 

 crease in size ; and at a later stage the mesoblast grows into 

 them along their axes, carrying the blood-vessels with it. During 

 the fourth week the villi grow actively ; they branch freely, and 

 in a very irregular manner. They penetrate the decidua, or 

 modified mucous membrane of the uterus, to a slight depth ; but 

 do not, as was formerly believed to be the case, grow into the 

 uterine glands. They become attached to the decidua at their 

 tips, but remain free along the rest of their length. As in their 



