THE THIRTEENTH AND FOURTEENTH DAYS. 



479 



the head and the yolk-sac. There is as yet no trace of either 

 visceral arches or clefts ; and the dorsal surface of the embryo is 

 enveloped in the thin membranous amnion, ajs', which now lies 

 rather closer to it than in the case of the embryo E. 



Human embryos of about the same age as the embryos B 

 and SR have been described by Allen Thomson (Fig. 177), 

 Keibel, V. Spee (Figs. 180 to 184), KoUmann (Fig. 185), and 

 others. These all agree in essential respects, and leave no doubt 

 that the stage must be regarded as a perfectly normal one. 



V. Spee's embryo, which was studied by means of sections, is 

 of considerable importance, as it has shown the internal struc- 



FlG. 180 



Fig. 180. — A Human Embryo of about the thirteenth day, from the left side : 

 the wall of the blastodermic vesicle has been in chief part removed. 

 (After V. Spee.) x 8. 



Fig. 181. — The same embryo from the dorsal surface. (After V. Spee.) x 14. 



AN", inner or true amnion. HD, head end of embryo. M"G, neural groove. NT , 

 neurenteric canal. PS, primitive streak. 'V'N, villi of chorion. YS, yolk-sac. 



ture, and the relations of the germinal layers, at the stage in 

 question. 



This embryo is represented from the left side in Fig. 180, and 

 from the dorsal surface in Fig. 181. It is, if anything, slightly 

 younger than the embryo E, and the constriction separating the 

 embryo from the yolk-sac has hardly commenced to form. The 

 head of the embryo (Fig. 181, hd) is wide and flat, and the 

 neural groove is shallow. At the hinder end, the two neural 

 folds diverge from each other, and embrace between them the 

 anterior end of a well -marked primitive streak (Fig. 181, Ps) ; 

 and just in front of the primitive streak is a small but well- 



