250 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



core of the rod in such a preparation does not show fihrillae, but a gran- 

 ular condition. Thus the mantle is shown to be more resistant than 

 the core. If the decolorizer acts too long, the core appears as a clear, 

 homogeneous, highly refractive substance, which does not stain, while 

 the mantle takes a deep red color. Since the process of depigmentation 

 works destructively on the cytoplasm, the finer details in such sections 

 can, of course, have little significance. 



At the level of the base of the rods sections tangential to the retina 

 which have been prepared in the ordinary way occasionally give the 

 impression that the sensoiy cells are pigmented in that part. That is, 

 one sees the mantle of the rod clearly, but what appears to be the core 

 is entirely obscured by pigment. The matter is easily explained by a 

 glance at Figures 7 and 8 (Plate 1), which are from depigmented ret- 

 inas. These figures represent two cross sections of the same cell-group 

 at the distal limit of the pigment. Figure 7 is 6| fx proximad to Figure 

 8, and contains just a little of the red-stained mantle on one side, the left 

 in the Figure. Here are five pigment cells arranged around the rod cell 

 in the typical way. Figure 8, cutting through the extreme tips of the 

 pigment cells, shows the same group surrounded entirely by a red-stain- 

 ing mantle (ivlr. bac), but at two places between pigment cells there are 

 small intercellular spaces which communicate with the vitreous humor. 

 Figure 2 shows that the pigment cells next to some of the rods extend a 

 little farther distad than their neighbors, a fact which gives the distal 

 boundary of the pigment zone an irregular course. Since the mantle 

 extends down over these protruding pigment cells, there is in some sec- 

 tions an apparent pigmentation of the sensory cell. 



Although sections 3J or 6§ micra in thickness show something of 

 the narrow part of the sensory cell which lies in the pigment zone, the 

 presence of the pigment is a decided hindrance to study. With the 

 pigment eliminated by a decoloring method, the sections may be made 

 thicker, with the result that the form of this part of the cell can be seen 

 better. That part of the sensory cell which lies in the pigment zone has 

 a form which is the counterpart of that of the core of the rod (Fig. 14) ; 

 that is, its funnel-shaped distal end matches the base of the core, and 

 a narrow stem extends through the pigment zone proximally toward the 

 large basal part of the cell. Judging from cross sections through this 

 region (Figs. 3, 5, 6, Plate l), the ensheathing pigment cells seem to 

 compress this part of the sensory cell into a prismatic form. There is, 

 however, much variation in the form and size of the sensory cells, es- 

 pecially in the peripheral zone, where they are distorted by the pressure 



