SEPARATION OF EMBRYO 



53 



fold first appears (fig. 79), and as a result of the forward growth of the front end 

 of the axis a diverticulum of the yolk-sac is formed. This becomes in part the 

 pharynx, but the fore-gut, as the diverticulum is called, is gradually lengthened by 

 the deepening of the fold and the coming together of the splanchnopleuric folds, 

 which are nipped in from each side (fig. 76, I.). In consequence of the formation 

 of the anterior fold, the buccopharyngeal membrane becomes bent in under the 



n.f. n.gr 



//Kv. ^ *%,\ 



III. 



FIG. 77. TRANSVERSE SECTION THROUGH 

 A HUMAN EMBRYO OF 2'4 MM. 

 (T. H. Bryce.) 



am, amnion ; n.gr., neural groove ; 

 not, notochord ; f.g., fore-gut ; y.s., yolk- 

 sac ; a, aorta of right side ; a.v., allantoic 

 vein of left side : c, ccelom. 



Vessels are seen covering the whole 

 surface of the yolk-sac. 



not.pl 



FIG. 76. TRANSVERSE SECTIONS OP THE HUMAN EMBRYO OF 2 MM. REPRESENTED IN FIG. 72. 



(After Graf v. Spee.) 

 In I., which is most anterior, the fore-gut is separated off from the yolk-sac. 



n.gr., neural groove ; n.f., neural folds ; ti.pl. (in III.), neural plate ; mes 1 , intra-embryonic mesoderm 

 still undivided : the commencing intra-embryonic coelom shows as a space (p) in I. to the left, and in 

 II. on both sides ; it becomes the pericardium; am.ect., amniotic ectoderm ; mes-, amniotic mesoderm ; 

 ent, entoderm of yolk-sac ; mes 3 , mesoderm of yolk-sac ; not.pl. (in III.), notochordal plate. 



head of the embryo and, reversed in position, now forms the still closed anterior 

 end of the fore-gut. Further, the pericephalic portion of the ccelom, also 

 reversed in position, comes to lie below the fore-gut, while the bridge of meso- 

 derm separating it from the extra- embryonic ccplom, and originally at the edge of 

 the shield, now forms the anterior lip of the primitive umbilical opening, and 



