484 



THE HUMAN EMBRYO. 



and the head end of the embryo is now more prominent, and is 

 raised distinctly above the yolk-sac. At the hinder end of the 

 embryo, to the left in the figure, the embryonal area still pre- 

 serves its primitive connection with the chorion or wall of the 

 vesicle. The head end of the embryo is covered over by the 

 commencing head fold of the amnion. 



Fig. 188 is a somewhat diagrammatic section, at a stage 



FIG. 187. FIG. 188. 



FlGS. 186-188. Diagrammatic longitudinal sections through Human Embryos, 

 representing hypothetical stages intermediate between Reichert's ovum 

 and His' Embryos, E or SR. (From His.) x 5. 



FlG. 1 86. Shows the commencement of the head fold of the embryo, and of the 



amnion. 

 FIG. 187. A rather later stage, in which the embryo is depressed into the 



blastodermic vesicle, but still remains in connection with the wall of the 



vesicle through the allantoic stalk. The dotted lines indicate, hypotheti- 



cally, the further growth of the amnion. 

 FlG. 188. A later stage, equivalent to that of His' embryos E or SR (Fig. 



179). The amnion is complete, and the villi extend the whole way round 



the vesicle. 



Am, inner layer of the amnion, or true amnion. S.Cft, dotted line representing the 

 future extension of the outer layer of the amniou. Ys, yolk-sac. 



corresponding to that of the embryos E or SR (Fig. 179). The 

 changes necessary to derive it from the stage shown in Fig. 187 

 are very slight. The hinder end of the embryonal area now 

 forms the thick allantoic stalk, connecting the embryo with the 

 chorion ; and a tubular diverticulum of the ventral wall of the 

 hind gut, the allantois proper, now extends some way along the 

 stalk. The amnion extends over the whole back of the embryo ; 



