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.