488 



THE HUMAN EMBEYO. 



Below the neck, in the angle between the embryo and the 

 yolk-sac, is the heart, H, a large tube sharply twisted on itself. 

 Blood-vessels are present in the wall of the yolk-sac, YS, and 

 also in the allantoic stalk, from which latter they pass into the 

 wall of the blastodermic vesicle, the inner layer of which is vas- 



A.s 



Pig. 196. — Human Embryo at the commencement of the third week. (From 

 His, after Coste.) x 15. 



A, inner or true amnion. As, allantoic stalk. 7/, heart. V, blood-vessel of yolk-eac. 

 I's, yolk-sac. 



cular throughout its whole extent, although the blood-vessels do 

 not penetrate into the villi. 



The middle portion of the embryo is clearly divided into 

 protovertebrae, but there are no traces of limbs. 



Embryo lettered by Professor His, Igr (Pigs. 189 and 197), and 

 estimated as fifteen days old. The entire blastodermic vesicle 

 in this case measures 17 by 11 mm., and is covered with villi, 

 except at two patches at opposite poles of the vesicle. The embryo 

 (Fig. 197) is rather closely invested by the amnion, AN, and is 

 connected with the wall of the vesicle by a short thick allantoic 

 stalk, TZ. The yolk-sac, ys, is nearly spherical, and about 2 mm. 

 in diameter, and is still widely continuous with the ventral 

 surface of the embryo. 



The most marked feature in the general form of the embryo 

 is the very sharp bend in the middle of the back, opposite the 

 yolk-sac. A similar and equally sharp bend has been noticed 

 in other embryos of about the same age (cf. Figs. 190, 193), but 



