The structure of the nuclei of the prism-cells is that of a dense network (Figs. 4, 7, npr) which stains dark with hæmatoxylin. A nucleolus can often be seen in these nuclei. In some few series, again, these nuclei did not show a network-like structure, but the chromatin was arranged in masses (Fig. 5, npr). These nuclei can usually be distinguished from those of the other cells of the retina by their denser, darker-staining network (Figs. 4, 7, npr), or as shown in Fig. 5 (npr). Their denser structure and staining capacity are a distinguishing characteristic of the nuclei of the prism-cells. I must add, however, that not in every series is this apparent. That portion of a prism-cell that contains the nucleus rarely contains any pigment; and when pigment is present, I believe that it has been dissolved in from the pigmented zone. The nucleus, again, lies a little centrad from the pigmented part of its cell, so that an unpigmented zone is seen in the retina between the pigmented zone and the row of nuclei (Figs. 4, 7, 22). Centrad the prism-cells are continued as a single process (Figs. 6, b, c, d, and 8a, b, c, d). In some sections I thought I could trace these processes to the basement membrane, but I could not satisfy myself that such appearances were not due to artificial splitting in the tissue. Schewiakoff makes a similar remark about his supporting cells, which cells I believe are the same as my long pigment cells, but these do not extend to the supporting lamella. At the margin of the retina the cells do not develop prisms but remain pigmented and form an iris (Fig. 7), which was so named by Claus and also described by Schewiakoff. These cells also assume a somewhat different shape (Fig. 6a). This cell (Fig. 6a) is seen from its broader side with which it is applied to the capsule or the lens. Schewiakoff figures similar cells. That the cells of the iris are prism cells without the prisms does not necessarily follow. They simply represent cells of the retinal cup that have become differentiated to serve as an iris. 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]