The Nervous System of Amblystoma. 449 



vations agree entirely with his. It will be assumed then in any 

 further consideration of this subject that the generally accepted 

 position in regard to the identity of the neural crests at the vari- 

 ous stages is correct. 



The relation of the neural crests to the neural plate as pic- 

 tured in fig. 2 at once suggests the hypothesis of the longitudinal 

 zones of the brain which has been elaborated in detail by John- 

 ston ('02, '05), but this particular theory of the neural crest 

 is not supported by the conditions just described. From the 

 fact that in Amphioxus the greater part of the nerve elements 

 homologous with the neural crest lie inside the neural tube and 

 that many of these elements are found there in fishes and even 

 in Amphibia, Johnston draws the inference that the neural 

 crest was originally a part of the central nervous system and 

 that there "has been a progressive separation of material for 

 the ganglia from the brain." It has just been shown, however, 

 that the neural crest develops at a later period than the neural 

 plate and that the two structures are separated by an appreciable 

 distance. Furthermore it will presently be shown that the 

 crest and plate are not segmented in the same way, a fact already 

 noted by Locy ('95). The evidence from the embryology of 

 Amblystoma then does not show a close relation between neural 

 plate and neural crest but rather it seems to indicate that the 

 plate represents the anlage of the central nervous system of the 

 primitive vertebrate and that outlying structures must have a 

 different homology. This position is further strengthened by 

 a study of neuromeres and sense organs, as will presently appear. 

 It will be shown to have an important bearing on the conception 

 of the structure of the primitive vertebrate head. 



Another important event which occurs in this stage is the 

 formation of a new groove, the neural groove, in the region first 

 occupied by the germinal depressions. This groove can readily 

 be distinguished from the posterior germinal depression by the 

 fact that the latter at this time has either entirely disappeared 

 or is fast degenerating while the new groove is deeper and sharper 

 than the old grooves and is much narrower than the posterior 

 depression (ng, fig. 2, D). In Plate I, figs. 4, 5, 6, the posterior 



