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TEXT-BOOK OF EMBRYOLOGY. 



somewhat elongated in the direction of the furrow. At the sides and ends the 

 disk is bent ventrally so that a depression is formed around it. The margin of 

 the disk is continuous with the amnion and with the yolk sac (Figs. 85 and 90, 

 B, C). The disk as a whole shows a trace of constriction from the yolk sac, 

 but at one end remains attached to the chorion by means of a mesodermal 

 structure — the belly stalk (Fig. 85) . 



Still a little further advanced than von Spee's embryo, is one described by 



Cerebral plate 



Heart 



Ant. entrance to 

 prim, gut (Ant. 

 intest. portal) 



Amnion* 



Post, entrance to 

 prim, gut (Post, 

 intest. portal,) 



\ Yolk sac 

 / (cut edge) TOl, \ 



Yolk sac 



Neural tube 



Belly stalk 



■ Neural fold 

 Neural groove 



Neural fold 



Fig. 119. — (a) Ventral view; (6) dorsal view of hu-.-ian embryo with 8 pairs of primitive 



segments (2.11 mm.). Etemod. From models by Ziegler. 

 In b the amnion has been removed, merely the cut edge showing; in a the yolk sac has 



been removed. 



Eternod (Fig. 119). What was originally the embryonic disk has here become 

 more elongated, and has assumed a sort of cylindrical shape owing to the rolling 

 under of the lateral margins. As a part of the rolling under process, the depres- 

 sion which originally surrounded the disk has become deeper and has effected a 

 still greater degree of constriction between the cylindrical body and the yolk 

 sac. The caudal end of the body remains attached to the chorion by means of 

 the belly stalk. The lips of the neural groove have turned dorsally and fused in 

 the middorsal line along part of their course. 



