Regression of the Neureiiteric Canal along the Streak. 



41 



space they enclose equals in width the lateral extent of the open 

 portion of the blastopore. Indeed, these two depressions are the 

 posterior extensions or horns of the crescentic portion of the open 

 blastopore. As this process of sinking continues, the central area 

 by degrees falls lower and lower, until it reaches the level of the 

 floor of the notochordal canal, at which level it disappears. That 

 portion which is going to form the dorsal wall of the archenteron 

 passes laterad and upward to fuse again in the dorsal lip, while 

 that portion which forms the floor of the potential archenteron will 

 disintegrate in order to put this cavity into communication with 

 the yolk. 



These successive stages in the sinking of the streak behind the 

 neurenteric canal, and its subsequent elevation and refusion an- 

 terior to this canal can be traced in a series of sections made 

 through the streak of an embryo whose blastopore has become 



VIII 



Diagrams II.-VIII. 



a crescentic opening with horns directed caudad. Diagrams II.- 

 VIII, are intended to illustrate this process. Diagram VIII. passes 

 through the streak behind the posterior extremities of the blasto- 

 poric horns. A primitive groove may be evident in this region, 

 but in some cases only a fusion of layers exists. (See Mehnert, 

 '92, Taf. XIX. Fig. 25 ; Mitsukuri, 'm, Plate III. Fig. 8; Will, '93» 



