480 



THE HUMAN EMBRYO. 



defined neurenteric passage, NT, leading from the surface into the 

 cavity of the yolk-sac. 



The sections (Figs. 182-84) will render the relations of the 

 parts clearer. Fig. 182, which passes across the anterior part of 

 the head, shows the widely open neural groove, XG ; the fore-gut, 



NT 



AN 



CH GT 



FIG. 183. 



FIG. 184. 



FIGS. 182-184. Sections across the Human Embryo of the thirteenth day, 



represented in Figs. 180 and 181. (After v. Spee.) x 45. 

 FIG. 182. Transverse section across the head end of the embryo. 

 FIG. 183. Transverse section across the middle of the body. 

 FIG. 184. Transverse section across the hinder end of the embryo and the 

 yolk-sac, the section passing through the neurenteric canal. 



AN", inner or true amnion. CH, commencing notochord. E, epiblast of the 

 embryo. GF, fore-gut. QT, mid-gut. H, liypoblast. M, niesoblast. ME, sornato- 

 pleuric layer of mesoblast. MH, splaiiclmopieuric layer of mesoblast. NG-, neural 

 groove. NT, neureuteric canal. YS, cavity of yolk-sac. 



GF, lined by hypoblast, and just shut off from the yolk-sac, YS ; 

 and the mesoblast, with its contained blood-vessels. Fig. 183 

 passes through the body region, and shows the commencing 

 notochord, CH, as a thickened plate of hypoblast cells in the 

 mid-dorsal wall of the gut. 



Fig. 184 passes through the neurenteric canal, NT, which 



