[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.
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 polygonal areas differing among themselves in size, but not showing two types of size, or two kinds of pigmentation, the one uniform, the other a ring of pigment around a highly refracting central portion. If the retina is but slightly pigmented—and some were so light as to make depigmentation unnecessary—a difference is seen in the pigment, as shown in [Fig. 63], but in no case were areas found that showed a highly refracting centre surrounded by a ring of pigment. (The unexplained structures in [Fig. 63] will be referred to a little later.)
[Figures 59-62] are a series of four successive sections drawn with the camera lucida for comparison with Schewiakoff’s Figs. 20 and 19, and to show that the presence of two types of cells plainly marked within the retina by the position of the nuclei at different levels is at least not clearly demonstrated. Only the nuclei are drawn, since the cell bodies are not easily distinguished from the surrounding fibres. The eye is the same as that from which [Fig. 72] was made. [Fig. 59] shows a relatively small number of nuclei of slightly larger size than usual. These I take for two reasons to be nuclei of the ganglion cells that are found in the fibres at the base of the retinal cells (Figs. [58], gc, [69] and [72]). They are the first nuclei struck in tracing sections toward the retina, and in the series from which [Fig. 58] was taken similar nuclei appeared in both transverse and radial cuts through the retina stained brightly and clearly with hæmatoxylin, whereas the nuclei of the retinal cells proper were stained a diffuse brownish-yellow from pigment that had evidently gone into solution. [Fig. 60] shows the closely aggregated, smaller nuclei of the retinal cells surrounded by the nuclei of the outlying ganglion cells. Schewiakoff’s corresponding drawing (’89, Fig. 20) shows at this level a definite alternation of the bodies and nuclei of unpigmented visual cells, with the smaller, pigmented, proximal processes of the pigment cells. In the next section ([Fig. 61]) the pigmented ends of a few of the cells have been struck, and the following section ([Fig. 62]) shows that, in this heavily pigmented specimen at least, there is no good evidence within the retina itself of two kinds of cells, so that it is apparent that at any rate we cannot accept Schewiakoff’s conception of the structure.
(b) Yet the fibres that Schewiakoff observed and associated with special visual cells occur beyond question. [Fig. 64] is a drawing of the first cut through the vitreous body of Charybdea, and in among the sections of the pigment streaks are seen sections of processes lying within clear spaces exactly as Schewiakoff figures his visual fibres (’89, Taf. II, Fig. 18). That the fibres occur is indisputable, but as to the cells to which they belong I can say nothing except that from such evidence as I have given in the preceding paragraph I conclude that they come from pigmented retinal cells of not very different type within the retina from the others, if different at all.
(c) On the third point, that the pigment streaks in the vitreous body belong to underlying cells and are continued distally into fibrous processes like the visual fibres of Schewiakoff, the evidence is decisive. [Fig. 58] has already shown it, and if this were not enough, a case of unusual stoutness of the fibres drawn in [Fig. 67] is conclusive. The preparation from which the section is taken was one preserved with corrosive-acetic, and I have drawn the outlines with the camera in order to avoid exaggeration of the fibres as far as possible, and also to show the shrinkage of the vitreous body (vb). It is the shrinkage of the vitreous body that makes it so difficult to determine the exact relation of structures seen in the vitreous body to the retina. The fibrous processes run through the vitreous body to the “capsule” of the lens (cp) (see also [Fig. 72]), a layer of homogeneous substance much resembling that of the vitreous body, which is classed as a part of the vitreous body, but usually in the shrinking adheres to the lens. The capsule is therefore regarded by Schewiakoff as a secretion of the lens cells. Some fibres were found by him to have the appearance of branching upon reaching the surface of the capsule, others of passing through it and of seemingly ending among the cells of the lens. The same appearances were given in my sections. It is altogether impossible in the distal portion of the vitreous body to distinguish between the fibres of Schewiakoff and those that come from the long pigment cells. ([Figs. 64-66] represent the appearance of the vitreous body at successive levels, and are from the same series of sections as [Figs. 59-62] and [72].) In Fig. 64 the sections of the processes that Schewiakoff calls visual are easily distinguished from the sections of the long pigment cells. In [Fig. 65], which is two or three sections nearer the lens, the pigment cells are shown by their cross-sections to be tapering down, and in [Fig. 66], nearer still to the lens, the two kinds of processes are no longer to be distinguished from each other. In a few cases I have found pigment in a fibre which but for this would be called one of the visual fibres of Schewiakoff. Such considerations as these, the similar appearance in cross-section, the finding of pigment in a few cases, and the inability to trace to any readily distinguished special type of retinal cell, make me wonder whether the visual fibres of Schewiakoff are anything more than the distal processes of pigment cells, into which the pigment granules happened not to be produced at the moment of fixation.