464 Joiiriial of Comparative Neurology and Psychology. 



As to the fate of the cells, it may be said that as soon as 

 the cell enters the canal, it becomes rounded and the nucleus 

 undergoes chromatolysis and disintegrates, followed by the 

 remainder of the cell, the cell membrane being the last struc- 

 ture to vanish. The cells most distant from the neural wall are 

 the first to disintegrate and are in some way absorbed, leaving 

 a clear central space. This process continues until all the cells 

 have disappeared and the clear space is identical with the cen- 

 tral cavity of the neural canal and the optic vesicles. 



A word might be added in regard to the development of 

 the optic vesicles in the Dipnoans (3, p. 439), the Ganoids (i, 

 P- 365) and the Teleosts (2, p. 185 ; 5, p. 243). In these, 

 the eye develops as a solid outgrowth from a solid neural tube, 

 and a cavity subsequently arises either by a splitting apart of 

 the cells, or by an actual absorption of those lying most cen- 

 trally. 



In the Amniota, (4), on the other hand, e. g. birds and 

 mammals, the optic vesicles develop as hollow projections of 

 the wall of the neural tube, whose cavities subsequently become 

 obliterated by the approximation of their walls. 



The process observed in Necturus conforms to neither 

 type. As shown above, the walls of the optic vesicle increase 

 rapidly in thickness until a certain maximal stage is reached, 

 after which they undergo a thinning until practically only a sin- 

 gle layer remains. During this period, the peculiar cells de- 

 scribed above appear in the lumen and undergo disintegration. 

 The wall of the optic vesicle at the time it begins to bud off 

 from the main neural tube encloses a cavity which is filled with 

 cast off cells from its own wall, on the way to disintegration. 

 Once free from its contents, the vesicle proceeds to develop 

 without again showing these structures. 



It is thus seen that in its development, the eye passes 

 through a stage that certainly recalls the condition found in so 

 many of the Anamnia. Whether or not this is to be consider- 

 ed of phylogenetic significance must remain an open question 

 for the present. 



The specimens on which the preceding observations were 



