The Nervous System of Amblystoma. 459 



front of the mid-brain and one behind the cerebellmn, and in 

 the closed tube of the chick the constrictions in front of and 

 behind the mid-brain are deeper than the others. The groove 

 behind the cerebellum in the trout may possibly be homologous 

 with the transverse cephalic groove of Amblystoma, but the 

 peculiarities of the formation of the brain of the former make 

 such comparisons of little value. These observations, then, 

 while they furnish proof of the actual segmentation of the neural 

 plate, emphazise particularly the great importance of the pres- 

 ence of such a land mark as the transverse cephalic groove of 

 Amblystoma. 



Scarcely less important than the neuromeres are the anlagen 

 of the lateral eyes. These appear as a pair of oval pigmented 

 depressions lying between the neural plate and the neural crest 

 (rs, fig. 4, B, fig. 5, E). In Amblystoma, although the retinal 

 spot is unquestionably present in a small proportion of the 

 embryos, it is not nearly so prominent as in Necturus and Rana 

 palustris. In the last named form it is very prominent in all 

 eggs of the open neural plate stage, occupying exactly the same 

 position as in Amblystoma between the neural plate and the 



crest. 



In the review of the literature on this subject it was shown 

 that the first observers of this retinal spot Whitman, Eycleshy- 

 mer, Locy, Hill were not specific as to its exact position in rela- 

 tion to the edge of the neural plate. They have described the 

 spots as being located in a general way on the plate, not beside 

 it. In Amblystoma the retinal spots are very clearly located 

 just lateral to the neural plate in the peripheral groove. If the 

 proposition that the neural plate represents the ancestral brain 

 be granted, then the eyes were located not on or in the brain, 

 as some authors have claimed, but just lateral to it in a position 

 corresponding to that found in some of the higher invertebrates. 

 A careful consideration of this question of the position of the 

 retinal spots is important in the understanding of the history 

 which follows in succeeding stages. 



As the neuromeres form on the procephalic lobes the neural 

 crests in this region become scalloped on their inner surfaces 



