478 THE HUMjLN embryo. 



is covered over its whole surface by branched villi, as ia the case 

 of the embryo E (c/. Fig. 1 88). The embryo (Figs. 1 78 and 1 79) 

 is 2*2 mm. long : it is attached to the inner surface of the 

 vesicle by a short thick stalk, TZ, and is separated from the yolk- 

 sac, YS, by a slight constriction. 



In the embryo itself, the head end, HD, is more markedly 

 raised above the yolk-sac than in the embryo E ; and the neural 

 groove, NG, is widely open along its whole length. The dorsal 



Fig. 179.— Human Embryo lettered by Professor His, SR, and estimated as ot 

 the thirteenth day. The wall of the blastodermic vesicle has been removed, 

 except the part to which the allantoic stalk is attached. (After His.) x 25. 



A-Ny inner or true amnion. HD, head end of the embryo. R, heart. UG, neural 

 groove. Til, tail. TZ, allantoic stalk, connecting the embryo with the -wall of the 

 blastodermic vesicle, VW, villi. YS, yolk-sac. 



surface of the embryo is somewhat sinuous in outline, present- 

 ing alternate convexities and concavities : the most anterior 

 and largest swelling is formed by the head ; then comes a con- 

 cavity opposite the middle of the length of the yolk-sac ; and 

 then another marked convexity farther back. The hinder end 

 -of the embryo projects freely as a short blunt tail, TL, beyond 

 the stalk of attachment to the wall of the vesicle, TZ, which now 

 arises from the ventral surface of the hinder end of the embryo. 

 The two halves of the heart, R, form prominent swellings between 



