The Nervous System of Amblystoma. 447 



figures illustrating this stage show, the changes which develop 

 in this groove consist of a widening and deepening and a growth 

 backward toward the blastopore. 



Lying in the peripheral groove at a point opposite the pos- 

 terior end of the anterior depression is a pit or short groove 

 (teg, fig. 1, E, F), one on each side of the neural plate. This 

 pit is the beginning of the broad transverse cephalic groove 

 which later marks off the anterior end of the neural plate from 

 the posterior end. It is visible in a majority of the eggs as soon 

 as the peripheral groove appears but in a few instances as in 

 fig. 1, D, no trace of it appears in this stage. Not onlj^ is the 

 anterior end of the plate, the procephalic lobes as this region 

 may be called, marked off by the groove but also, when the eggs 

 are hardened in chromic acid, by a striking difference in color. 

 The procephalic lobes are darker than the remainder of the 

 plate, a distinction which remains until about the time of the 

 closing of the neural tube. Thus at a very early stage color and 

 a definite boundary mark out the region which proves to be 

 the anlage of the brain. The transformation of this anlage, 

 the procephalic lobes, will be the principal object of attention 

 in the succeeding stages. 



Stage 5 (Figs. 2, 3, Plate I, figs. 6, 7). This stage is charac- 

 terized by the appearance of the neural crest (nc, fig. 2). The 

 crest first appears as a pair of short longitudinal thickenings 

 at the sides of the neural plate opposite the transverse cephalic 

 groove (fig. 2, E). Each half of the crest grows rapidly back- 

 ward until it reaches the blastoporic rim (br, fig. 2, A) which 

 in a few eggs is still distinctly visible. After the disappearance 

 of the blastoporic rim the two sections of the crest fuse behind 

 the neural plate (fig. 2, B). At a slightly later period the crest 

 is continued around the anterior end of the plate (fig. 2, C). 

 This description of the origin of the neural crest agrees with the 

 more general statement of Eycleshymer ('95) that in the Am- 

 phibia the neural ''bands arise independently" and ''differen- 

 tiate in situ." 



It has been quite generally assumed that the neural crest 

 which surrounds the open neural plate is identical with the later 



