A DESCRIPTIVE SKETCH 



27 



§5 

 The neural folds now rise up as a pair of parallel ridges along the 

 dorsal side of the embryo, and come to enclose the blastopore, 

 which is now reduced to a mere slit, between their hind ends. As 

 soon as this has happened, the embryo may be termed a neurula. 

 The groove between the neural folds becomes converted into a tube 



4 5 6 



BO. p. 

 C d 



Fig. 8 

 The process of neurulation in Urodeles, revealed by the movement of intra vitani 

 stain marks placed on the surface of the gastrula in a transverse line across the 

 animal hemisphere, a, Seen from the dorsal side, b, From the right side, c, With 

 the progress and completion of gastrulation, the band of stain marks becomes 

 U-shaped, the arms parallel with one another along the dorsal side, and marking 

 the site of formation of the neural folds {d). (After Goerttler, Arch. Entwmech. 

 cvi, 1925, modified.) 



as the neural folds arch over to join one another and fuse above it, 

 and the blastopore is no longer at the surface of the embryo, but is 

 covered over by these folds. In this manner, a neurenteric canal 

 (actual or virtual according as to whether the blastopore is or is not 

 still open) is formed, connecting the cavities of the neural tube and 

 of the gut. After the fusion of the neural folds, epidermis covers 

 the entire surface of the embryo, and the rudiments of all the other 



