40 



HUMAN ANATOMY. 



Between the points a, b in the figure Hes the area through which the embryonic 

 growth has made its way into the mucous membrane of the uterus, and, in consequence, 

 the uterine epithehum in this area has disappeared. Above this small area there 

 lies a covering mass of tissue ( T. M. ) mainly composed of blood, the result evidently 

 of hemorrhage following the breaking of the mucosa of the uterus in this region*. 

 The chorionic vesicle as a whole is quite large, especially in proportion to the 

 embryonic area E, the surface of which is covered with a distinct columnar epithelium. 

 Surrounding the chorionic vesicle there are two kinds of tissue, which make a very 

 striking' feature of the picture. F'irst, there is the thickened and very irregular 

 trophoblast, the cells of which appear dark, and which forms the outer covering of the 

 wall of the embryonic vesicle itself. Then there are numerous large blood-spaces or 



Fig. 47. 



/ 



Amnion - 



Umbilical or yolk-sac 



V 



Chorion 



Human embryo of about twenty days, enclosed within the amnion. X 30. 



blood-lacunse lying among the irregular projections of the trophoblast. The maternal 

 blood, therefore, in this very early condition bathes the trophoblast cells of the 

 embryo, a relation very significant with reference to the nutrition of the embryo 

 before the allantoic-placental circulation is established. The mesoderm (Af) extends 

 around the vesicle on the inner side of the trophoblast. In several places there are 

 outgrowths of the mesoderm into the trophoblast, so indicating the beginnings of the 

 villi of the chorion. It will be remembered that the cells of the trophoblast form the 

 epithelial covering of the chorion. At several places in the figure the syncytial 

 layer of the trophoblast Sj can be distinguished. The proportionally large cavity 

 within the vesicle is extra-embryonic coelom, a fact which can readily be verified by 

 observing the relations of the mesoderm. The latter layer of tissue is seen to extend 

 around the small yolk sac as the visceral layer of the mesoderm, while the layer of the 

 mesoderm on the inner side of the trophoblast is of course the parietal layer, hence 

 the cavity within these respective layers is the extra-embryonic coelom, precociously 

 developed for this early stage. There is a small amniotic cavity above the embryo. 

 Between this cavity and the trophoblast the mesoderm extends as a solid sheet. 

 There are one or two more points to be noted in the figure. In the areas 



