The Nervous System of Amblystoma. 467 



anus as described above, next they meet over the posterior part 

 of the cephalic lobes and from these two points the process of 

 closing goes rapidly on. The anterior end of the canal remains 

 open for a brief period as the anterior neuropore (anp, fig. 9, B), 

 but the neuropore does not lie at the extreme anterior end of 

 the canal, for there is a slight concresence of the ridges in front 

 of the neuropore leaving a groove which is descernible in all 

 eggs of the right condition, and furthermore the blind end of the 

 neural tube always extends an appreciable distance in front of 

 the neuropore. These relations may be easily seen in the speci- 

 mens shown in figs. 8 and 9. 



This concresence of the fold in front of the neuropore is an 

 important factor in determining the anterior limit of the neural 

 axis. Kupffer and other authors have considered the anterior 

 border of the neuropore as the anterior end of the brain but 

 Johnston ('05) has more recently shown that this point owing 

 to a concresence, such as has just been described, varies greatly 

 in different types of vertebrates. Johnston's conclusion, however, 

 that the anterior end of the brain from a morphological point 

 of view lies at ''that point at which the brain plate meets the 

 general ectoderm at the same time tiat it comes in contact with 

 the anterior end of the endoderm" does not form a criterion that 

 can be applied to the conditions found in Amblystoma, for the 

 neural place does not come in contact with the general ectoderm 

 since the neural crest intervenes (fig. 8, A) . If the open segmented 

 neural plate be considered as the neural axis, or the anlage of 

 the neural axis, then the anterior margin of the first neuromere 

 constitutes the anterior limit of the nervous system, a point, 

 however, which cannot be traced into the adult brain of Ambly- 

 stoma although it is visible just before the closing of the neural 

 tube. On the other hand, if the anterior border of the neural 

 crest be taken as the point in question, on the ground that the 

 neural crest forms an integral part of the central nervous system, 

 at least in modern vertebrates, then the neural axis ends at a 

 point slightly in front of the neuropore, a point which in Ambly- 

 stoma can no more be determined with accuracy in adult stages 

 than the anterior limit of the neural plate. Thus Amblystoma 



