480 



THE HUMAN EUBKYO. 



defined nenrenteric passage, XT, 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 nenral groove, XG ; the fore-gnt, 



Fig. 183. 



Fig. 184. 



Figs. 182-184. — Sections across the Hnman Embiyo 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. — Transrerse 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 nenrenteric canal. 



AN, inner or true amnion. CBC, commencing notochord- E, epiblast of the 

 embryo. GF, fore-gnt. GT. mid-gut. H, hypoblast. M, mesoblast. tim , somato- 

 pleuric layer of mesoblast. TWi h , splanchnoplenric layer of mesoblast. If (x, neural 

 gtoore. IJ"T, nenrenteric canaL YS, cavity of yolk-sac. 



GF, lined by hypoblast, and just shut ofiF from the yolk-sac, TS : 

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

 passes through the body region, and shows the commenciDg 

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

 mid-dorsal wall of the gut. 



Fig. 184 passes through the nenrenteric canal, >t, which 



