[Fig. 63], however, where the retina was only slightly pigmented, rather speaks against this view, for the number of darkly pigmented areas seen here (which are shown beyond question by radial sections to belong to the long pigment cells) is not great enough to account for the number of both pigment areas and visual fibres of Schewiakoff seen in such a section as [Fig. 64]. This would throw the visual fibres of Schewiakoff back upon some of the slightly pigmented cells of [Fig. 63], otherwise not distinguished. I think the question cannot be settled without the maceration of fresh material, and experiments upon eyes killed in the light and in the dark.
In such cases as that of [Fig. 63] it would seem conclusively shown that the long pigment cells must belong to a different type from the short, but as I have already said I can find no regularity in either their shape or in the position of their nuclei. And on the other hand [Fig. 58] shows that the reverse relation may obtain and the long cells be less deeply pigmented on the edge of the retina than their shorter neighbors, so that it looks as if all the short cells had to do was to project half their pigment out into the vitreous body in order to become exactly like the long ones. This they could do if, as is possibly the case, they are prolonged into “visual fibres” of Schewiakoff that have escaped observation and so do not appear in the drawing.
[Fig. 58] shows one more thing that is worthy of remark in passing. In the preparation in which the vitreous body (at this point at any rate) was not shrunken away from the retina, the fibre from each long pigment cell does not lie in a clearly defined space or “canal,” such as is usually described as a constant structure of the vitreous body. Very likely these canals are formed only by shrinkage around the fibres, and the irregular shape of the spaces around the three fibres in [Fig. 67] rather bears out the same supposition.
As to the structure of the vitreous body, apart from the fibres and pigment streaks already mentioned, I find it to be made up of prisms extending from retina to capsule of lens, each containing a central axis or fibre. [Fig. 64] shows that the space around the pigment areas and “visual fibres,” instead of being homogeneous, is wholly filled with the polygonal cross-sections of these prisms. In Charybdea they are generally more difficult to perceive than in my best material of Tripedalia which was killed in acetic acid. In this the polygonal areas stood apart from each other more plainly. Curiously enough I have been unable to demonstrate in Tripedalia the “visual fibres” of Schewiakoff. Here and there were found spaces that at first sight reminded of them ([Fig. 69], sh), but they contained no central fibre, and were probably due to shrinkage. The polygonal areas themselves, however, often contained a clear spot in the centre, at one side of which would be found the cross-section of the fibre, as is shown in many cases in [Fig. 68]. The clear spot is here undoubtedly due to shrinkage of the gelatinous substance of the prism.
I think that these prisms and fibres are the direct continuations of retinal cells. In a section such as that drawn in [Fig. 63], which takes just the very tops of the cells of a slightly pigmented retina, in the centre of the section just grazing the space that lies between the retina and the shrunken vitreous body, most of the cells toward the middle (where especially the extreme tips are taken) show in their centres a dot exactly corresponding to the dots in the polygonal areas of the vitreous body. In the exact middle of the section, where only the cell walls appear, slightly indicated, a dot is seen in each case. The size and shape of the ends of the cells correspond with those of the polygonal areas in the vitreous body, and I do not doubt that the latter are continuations of the former. The vitreous body, then, instead of being homogeneous, is composed of the clear highly refracting outer ends of retinal cells. The assumption lies near that these are the true visual rods, but of course it is assumption only.
To give a brief review, the points in which my conclusions differ from those of Schewiakoff are as follows: I find (1) that the long pigment streaks are parts of retinal cells continued into processes like his visual rods; (2) that the vitreous body is composed of prisms with central fibres proceeding from retinal cells; (3) that I am unable to get satisfactory evidence of two types of cell distinguishable within the retina, and at any rate find considerable evidence against the two types he distinguishes.
These results are not wholly satisfactory, for they leave us with three kinds of fibrous processes in the vitreous body which for the present we are unable to trace to three, or even two distinguishable types of cell in the retina. It would be more pleasing if we could confirm Schewiakoff’s simple conception of the structure, with its one set of visual rods in the vitreous body referable to a clearly marked type of sensory cells in the retina, but I think the evidence that has been brought up justifies the conclusion that in some respects he saw too much, in other respects too little. This is not to be wondered at, since his material, to judge from a single statement, consisted of but twelve marginal bodies, and, moreover, the work on Charybdea forms but one portion of a paper that is excellent for the clearness of its descriptions and illustrations.
Before leaving the subject I must mention that Wilson suggested from his observations on Chiropsalmus that the vitreous body had a prismatic structure, but he was probably mistaken when he thought he found evidence of nuclei in it. Claus says that the retina is composed of pigment and rod cells alternating, and Wilson agrees with him, but under a sketch of a sense cell from the nerve he makes the express statement “not very well preserved.” It seems very probable, therefore, that he followed Claus’s interpretation rather than independent observations, and Claus interpreted his results very much by analogy of what had been found in other forms.
The smaller complex eye which is represented in [Fig. 69] agrees in structure very closely with the larger. The chief differences are that sections do not show pigment extending into the vitreous body, that there is no “capsule” to the lens, and that the lens seems to be supported by a kind of stalk formed by a thickening of gelatine of the supporting lamella (sl). The gelatinous thickening lies between the lens and an outgrowth of endodermal cells (en) from the canal of the club. This outgrowth is a constant feature, figured by Claus and Schewiakoff for Charybdea, and by Wilson for Chiropsalmus, and found in Tripedalia also. The regularity of its appearance in all three genera leads one to suspect that it may have some significance not yet understood.
Just above the smaller eye there lies a mass of cells of peculiar structure ([Fig. 69], nc). They are of a rounded polygonal contour, with a comparatively small circular nucleus in the centre, and are found in this region only. In and amongst them bundles of fibrous tissue are found in the sections, which pass from the surface cells to the supporting lamella. Claus describes the contents of these cells as coarsely granular protoplasm and says they cannot be taken for ganglion cells. He is inclined to believe that they play the part of a special supporting tissue. Schewiakoff, on the other hand, is convinced that they are ganglion cells, and finds processes passing out from them (’89, Taf. II, Fig. 22). I find, however, that the cell contours are perfectly regular and clearly without processes, and it is incomprehensible to me how, if his material was at all well preserved, he could for a moment have taken them for the same thing as the big multipolar ganglion cells with large nucleus and nucleolus which lie in about the same region and were correctly described and figured by Claus but are not specially mentioned by Schewiakoff. I cannot agree with Claus, however, that their contents are composed of coarsely granular protoplasm. That which appears such by low magnification shows itself under high powers to be a beautiful network with thickenings at the nodes of the meshes, which is brought out very plainly by a cytoplasmic stain such as Lyons blue. Around the nucleus is seen a more or less well-defined clear zone. What the function of the cell is remains as unknown to me as to Claus and Schewiakoff.