[Illustration: [Dr. Conant did not complete Fig. 72, and the accompanying outline of Fig. 7 of Schewiakoff’s memoir (Beiträge zur Kenntnis des Acalephenauges, Morph. Jahrb., Bd. XV, H. 1) has been substituted.--EDITOR.]
EXPLANATION OF LETTERS IN TEXT FIGURE.--_C_--concretion cavity; _CO_--cornea; _CP_--capsule of lens; _CSC_--cavity of sensory club; _EC_--ectoderm; _EN_--endoderm; _ENC_--endoderm of sensory club; _L_--lens; _NC_--network cells; _NF_--nerve fibres; _RT_--retina; _SLA_--supporting lamella; _VF_--vitreous body.]
Fig. 72 is a horizontal section through the large eye, and shows that here, too, when the sections pass through the eye just radially, the 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 between 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 supporting 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 Protoplasma des centralen Zellenabschnittes, sondern ist auf seine Oberfläche 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.