62 Journal of Comparative Neurology. 



pear-shaped than in the other parts of the cephalic plate. The 

 differentiation is by no means as marked as in the Rana palusr- 

 tris figured by Eycleshymer — nevertheless indications of change 

 are not only wholly lacking. My sections are too thick (iO/< 

 to 1 5/z ) for satisfactory histological study, but one can see in 

 them that the middle of the walls of the optic cups are areas of 

 differentiation. The differentiation does not progess far until 

 later, but the frequency of dividing cells, and their change of 

 form continue, in these early stages, to be marks for distinguish- 

 ing visual epithelium from that of the surrounding brain-walls. 

 The dividing cells are neuroblasts, and these are known to be 

 points from which the nerve-fibers spring; their presence in any 

 considerable number would therefore indicate a differentiation 

 in the direction of increase of sensibility." 



The elongation of cells in these areas would certainly be 

 indicative of differentiation, providing mechanical stresses be 

 excluded. The existence of mitotic figures in a rapidly growing 

 structure, such as the beginning of the infundibulum in which 

 the optic vesicles are at first located, or in the rapidly expand- 

 ing walls of the thalamencephalon in which the first pair of 

 accessory vesicles are incorporated, would not necessarily in- 

 dicate a differentiation in the direction of increase of sensi- 

 bility. That the epithelium is "visual" would also appear 

 questionable. 



It can be said with perfect certainty that even where the 

 optic cups are as well differentiated as in Rana palustris, one 

 cannot by any known method demonstrate the " visual char- 

 acter " of the epithelium. During the involution of the optic 

 cups, as the senior author stated in an earlier paper, "a marked 

 migration of the pigment has taken place : instead of being lo- 

 cated at the ends of the cells, as in the earlier stages, it is found 

 between them and nearer the periphery. The nuclei have like- 

 wise undergone a further migration toward the surface, so that 

 the cells of the superficial layer have completely lost their iden- 

 tity." Their fate is unknown. 



Should later investigation prove that in the unclosed neural 

 plate of Squalus there are present from two to eight pairs of 



