F. S. CONANT ON THE CUBOMEDUS^. 51 



nuclei are not found at different levels sufficiently definite to suggest two 

 kinds of cells. 



In the inner corner of the retina in the same figure (69) are seen 

 cells without pigment which show nuclei undoubtedly at different levels. 

 These cells in this position are a regular feature in the retina of the 

 smaller eye. Schewiakoff considers them purely visual, because of the 

 lack of pigment. In so doing it seems to me he forgets his own standard 

 for discriminating beween pigment and visual cells. The pigment cells 

 of the retina, according to him, are the same thing as the cone-shaped 

 supporting cells found elsewhere in the nervous epithelium, and are, 

 therefore, distinguished from the visual cells primarily by shape and by 

 position of nucleus, secondarily by the greater development of pigment. 

 When on the ground of pigmentation alone he calls the cells in the 

 corner of the retina visual, he judges them by only the second test, and 

 in so doing virtually admits, as it seems to me, that shape of cell and 

 position of nucleus are matters of no great moment. His own standards 

 place him in a dilemma. If on the other hand he judges by the lack of 

 pigment, the cells are visual ; if by shape of cell and position of nucleus, 

 they are both visual and pigment cells without the pigment or sup- 

 porting cells. What use there would be for simple unpigmented cells in 

 one limited region of the retina is hard to see, so he naturally takes the 

 other horn of the dilemma and calls them visual because they have little 

 or no pigment. 



The distinction, then, between pigment and visual cells is brought 

 down to one of pigmentation only. Schewiakoff s test for this is that in 

 the visual cells "Das Pigment durchsetzt aber nicht das ganze Proto- 

 plasma des centralen Zellenabschnittes, sondern ist auf seine Oberflache 

 beschrankt (Fig. 19, sz), so dass der innere, axiale, stark lichtbrechende 

 Theil vollkommen frei von demselben ist." ('89, p. 37.) That is, in a 

 section through the ends of the retinal cells each pigment cell will 

 appear as a uniformly pigmented area, while each visual cell will appear 

 as a light, strongly refracting spot with a ring of pigment around its 

 periphery. This is the arrangement given in his Fig. 19. 



An arrangement so definite ought to be easily made out in sections, 

 yet I have not been able to find it so. My sections show considerable 

 difference in the amount of pigmentation even in material preserved 

 with the same killing agent. If the retina is heavily pigmented the 

 ends of the cells have the appearance shown in Fig. 62, which represents 

 a portion of a cross-section. The ends are seen as clearly defined 



