As to the exact origin of the prisms, and pyramids (to be described below), it is difficult to say anything definite. If the so-called basal bodies of the axial fibers are really homologous with the basal bodies of flagella, then it would seem that they (the prisms and pyramids) are secretions comparable to cuticular secretions.
(b) The pyramid-cells, like the prism-cells, are differentiated into three regions: a distal vitreous pyramid, a pigmented part, and a centrad part with nucleus. The pyramids are seen in transverse section in [Fig. 1] (pyr) and in longitudinal section in [Figs. 4 and 7] (pyr).[e]
Each pyramid extends between the bases of the prism-cells about one-third to one-half the depth of the vitreous body (Figs. [4], [7], [12] (pyr)). The pyramids are also a shade lighter than the prisms, which fact is characteristic. In the axis of each pyramid is a darkly-staining fiber quite like the one described for the prism-cells (Figs. [1], [4], [7], [22]). That this fiber extends distally beyond the limits of the pyramids I could not determine, but I do not think that it does. Centrad this fiber extends into the pigmented portion of its cell quite to or near the nucleus as was described for the fibers of the prism-cells (Figs. [7], [22]). Whether or not these fibers extend past the nucleus and become continued as nerve fibers, the same course of reasoning holds as was given for the fibers of the prism-cells. Each of these fibers possesses a basal body just on its entrance into the pigmented part of the cell ([Fig. 7]), but I could not determine that it was dumbbell-shape. In form it represents an enlargement of the fiber itself, which gradually tapers again to its normal size. The continuations of these fibers within the pigmented parts of the pyramid-cells, as also the basal bodies, could only be demonstrated in unpigmented series.
Patten[5] describes axial fibers extending centrad through the rods (vitreous portions) of retinal cells (“retinophora”) into the region of the nucleus and past the nucleus (arthropods and molluscs). My retinal cells (prism and pyramid cells) evidently correspond to Patten’s retinophora, but I find no evidence that one of my retinal cells represents more than a single cell, while Patten gives evidence that his retinophora are made up of two cells closely applied to each other as twin cells. If this were also true for the retinal cells that I have described, I believe my macerated preparations would have shown it. Schreiner[12b] and Hesse[13] also figure and describe axial fibers for the rods of the visual cells in polychætous annelids, and Schreiner[12a] also for molluscs. Neither of these observers finds the fibers to extend distally beyond the rods nor centrad toward the nucleus as Patten and myself show. Neither Schreiner nor Hesse figures these cells as twin cells as Patten does, so that to my knowing Patten stands alone in this respect. Andrews[14] describes and figures rods for the visual cells of polychæte annelids but no axial fibers. He was the first to describe these rods in annelids.
The pigmented zone of the pyramid cells, in heavily pigmented series, is filled throughout with dark-brown pigment granules, and is quite like that of the prism cells ([Figs. 4, 7]). In transverse sections, however, through the most distal part of the pigmented zone, of unpigmented series ([Fig. 2]), lighter areas with central dots could occasionally be demonstrated, which areas are the pyramid cells. In [Fig. 2], the more definite polygonal outline as well as the lighter shade of these areas was a distinguishing feature. The difference in shade was not wholly due to a difference in pigmentation but to a structural difference.
The nuclei of these cells are usually a little larger than those of the prism cells and are filled with a finer and less dense network ([Figs. 4 and 7], npyr), in consequence of which they present a lighter appearance in sections when examined with a high power. It will be seen in the figures ([4, 7]) with what regularity these lighter nuclei lie opposite the pyramids. Some few exceptions occur. These are probably due to the fact that a nucleus or pyramid was not differentiated by the technique. If this opposition between the pyramids and the lighter nuclei were all, I believe it would be sufficient evidence for associating these lighter nuclei with the pyramid cells.[f]
(c) The long pigment cells are about as numerous as the pyramid cells. In these cells, as in the prism and pyramid cells, three regions can be distinguished: the region of the nucleus, a pigmented region (the distal half of which extends between elements of the vitreous body), and a distal rod-like portion, or fiber, which is continued between the prisms into the capsule of the lens ([Figs. 4, 7, 9]). The pigmented portion is about twice the length of that described for the other cells, and also often of greater diameter, so that in transverse sections ([Figs. 1, 2, 3]) these cell-areas are larger than those of the other cells. As nearly as I could determine, these cells are pigmented just like the other retinal cells described. In quite unpigmented series, however, they often contain more pigment than the other cells do ([Fig. 2]). Distally, the pigmented part becomes narrowed to a strong pigmentless fiber ([Figs. 3, 4, 7]). This fiber stains quite dark with iron-hæmatoxylin and appears homogeneous. It passes between the prisms into the capsule, where it usually bends in a direction toward the margin of the capsule ([Fig. 7]) and passes diagonally across this to the lens. In sections, a space is often seen about these fibers in the vitreous body, which I regard as a shrinkage space ([Figs. 3, 4]), since it is not evident in all series ([Fig. 1]). In [Fig. 7], I have assumed that these spaces are due to shrinkage and have not indicated them. Also, in this same figure I have assumed that the spiral appearance of the fibers ([Fig. 4]) is due to a shortening of the prisms during fixation, and have drawn them straight. At the lens these fibers seem to end. In a few instances they were seen to branch upon reaching the capsule ([Fig. 4]). In [Fig. 9], also, which shows some of these cells from a macerated preparation by Conant, the rods show evidence of branching at their distal terminations. In the same preparation I thought I could see that a fiber became expanded into a membrane spreading over one of the lens-cells. I could not satisfy myself, however, that this was the actual condition of things. Judging from [Fig. 9], one might conclude that all the fibers are branched distally; yet, if such were the case I should have seen more of it in sections, but branching as seen in [Fig. 4] is the exception. Hence, if all these fibers do branch, I am inclined to believe that it must be among the bases of the lens-cells. Or, if the fibers do expand into membranes to cover the lens-cells (I could not explain purpose), the evidence in [Fig. 9] may be nothing more than fragments of this membrane left attached to the ends of the fibers. As is seen in [Fig. 7], most of these rods end opposite the cells of the lens, and not usually between two adjacent cells as Schewiakoff has described for Charybdea marsupialis. The nuclei of these cells are like the nuclei of the pyramid cells ([Figs. 4, 5, 7, 9]) and often have a nucleolus.[g] Centrad these cells are continued into a number of processes as is seen in [Figs. 5, 7 and 9]. How far the several centrad processes extend and where they end I cannot say; but, as seen in [Fig. 5], they soon taper to a thin end which I suppose may be continuous with a nerve fiber. I believe Schewiakoff was mistaken when he stated that these cells extend to the basement membrane.
I have found no evidence in these cells of the existence of an axial fiber such as I have described for the prism and pyramid cells. I find no definite arrangement of the nuclei of the retina into definite layers, but the nuclei of the three kinds of cells lie quite mixed, sometimes one kind lying deeper than the other as can be seen in the figures. Again, they may lie quite at the same level. (This point will be referred to later.)
It is these long pigment cells that I believe retract their pigmented part from between the prisms and pyramids when the medusæ are placed in the dark, protruding with their pigment when placed in the light. [Fig. 5] is a section from a slightly pigmented retina killed in the dark. The parts of the cells projecting beyond the pigmented zone, and which would lie between the prisms and pyramids (here not shown) of the vitreous body are seen to be narrower than in sections from retinas killed in the light ([Figs. 1, 3, 4, 7]) and the cells themselves appear in a condition of retraction as is shown by their large centrad portions with the nuclei, which latter, also, here lie at quite a lower level than the other nuclei. (The pyramid cells were not shown in this series.) I occasionally found appearances like [Fig. 5] in retinas killed in the dark (indeed, in some the pigmented portions in the vitreous body were much thinner and more retracted than in [Fig. 5]). Yet this appearance was not of sufficiently general occurrence to leave no doubt as to its significance. As positive evidence, however, I cannot give it any other interpretation than the one given—that the cells retract themselves with their pigment when in the dark. Again, it must be added that the nuclei of these cells may occasionally lie quite deep even in retinas killed in the light. Indeed, like structures in different retinas may vary considerably in size and shape. None of my darkness retinas, however, showed such a large proportion of the pigmented parts of the long pigment cells projected between the prisms and pyramids as did the light retinas. I examined and tabulated all my series with respect to the extent the long pigment cells were projected into the vitreous body, and I found that those which showed these cells with their pigment least projected between the prisms and pyramids to be those that had been killed in the dark. I thus feel satisfied that the pigmented parts of these cells become in part or quite completely retracted from between the prisms and pyramids of the vitreous body when in the dark, but just how this is accomplished—whether the whole cell with its nucleus takes up a deeper position, the cell substance at the same time collecting in the region about the nucleus, as shown in [Fig. 5] and the diagram ([Fig. 22]), I cannot with certainty state. It would seem, too, as though the pigment became less in the cells exposed to darkness, for I rarely, even in the most retracted heavily pigmented series, saw the pigment to extend farther towards the nucleus than commonly. The time of keeping in the dark, prior to fixing, varied from three-fourths of an hour to one and one-half hours. I could not bring the amount of retraction into relation with the time of exposure, except that in general the retinas longest exposed showed the greater amount of retraction.