AMPHIBIA 



227 



endoderm at the anterior end, but it usually, if not always, 

 takes its place by the excavation of the intervening cells as the 

 front end of the enteron (fig. 115). 



The blastopore, as it narrows and closes at the posterior 

 end of the embryo, forms a vertical slit, a short primitive streak 

 and groove. 



As these changes are approaching completion the dorsal 

 ectoderm is thickened and flattened. This is the neural plate, 



Dorsal Up 



Ect. End. Ent. Dorsal lip 



Yolk plug 2 



Ventral lip Enteron Yolk plug 



FIG. 115. Longitudinal sections of postgastrula stages to illustrate the over- 

 lapping of the delaminated endoderm by the blastopore. The primitive 

 enteron, often called the segmentation cavity, is united to the secondary 

 or neurenteric enteron by an absorption of the cells intervening between 

 them. These figures are rotated about 90 compared with those of fig. 1 14. 



and it becomes further denned by the appearance of the neural 

 fold, which arises anteriorly, and transversely, to mark the 

 anterior limits of the plate and extends backwards on each 

 side until it reaches the middle of the primitive streak, thus 

 defining a neurenteric groove. It is further defined by the 

 appearance of a median groove which begins at the dorsal lip 

 of the blastopore and extends forwards in the mid-line of the 

 neural plate. The neurenteric groove is converted into the 

 neurenteric canal by the fusion of the neural folds, and the 

 fusion is continued forwards, with the result that the neural 



