RHINEURA. 53 



pigment cells are sometimes found in the inner layers of the retina among the gan- 

 glionic cells and along the optic nerve within the eye. Pigment cells were also 

 found in the eyes of Typhlomolgc (figs. 5 a and 7 a, z). There are rarely any 

 pigment cells over the distal face of the eye. 



(i a) X, nuclei. In the eyes of Rhiiieura, Typhlichthys, and Troglichthys a 

 few cells with elongated, tangentially placed nuclei are present between the pig- 

 mented epithelium and the outer nuclear layer. They are distinctly outside of 

 the outer limiting membrane (figs. 19 a, b; plate 3, fig. E, x). The origin of these 

 nuclei is difficult to explain. Possibly they are derived from the pigment epithe- 

 lium which in some of the unpigmented regions (fig. 19 b, x) is more than one layer 

 deep. If the outer layer should become pigmented, the inner nuclei, if they 

 remained unpigmented, might give rise to these longitudinal cells. 



(2) Rods and cones are not present. There is in some cases a distinct space 

 between the pigment epithelium and the outer nuclear layer. This space when 

 present is partially filled with filmy, hazy structures, but nothing suggesting defi- 

 nitely either a rod or cone was detected (fig. 19 a and plate 3, fig. E). 



(3) The outer nuclear layer consists of about 2 series of elliptical nuclei. They 

 form a compact and distinct layer a few microns from the outer limiting mem- 

 brane (figs. 18 a, 19 a, b, and plate 3, fig. E). 



(4) The outer reticular layer is represented by a series of distinct but irregular 

 gaps between the outer nuclei and the inner nuclei. Horizontal cells are not pres- 

 ent (figs. 19 a, b, c, and plate 3, fig. E). 



(6) The inner nuclei are smaller, rounded, and less granular than the outer 

 nuclei. They do not form as compact a layer as the outer nuclei. It is impossible to 

 distinguish between bipolar and spongioblastic cells (6 in figs. 18, 19, and plate 3). 



(8) The inner reticular layers, as is usual in degenerate eyes, are well developed 

 in the eyes of Rhineura. They are frequently crossed by Miillerian fibers. 



(9) The ganglionic layer is represented by a number of nuclei loosely grouped 

 about the vitreous slit. The individual nuclei are distinctly larger than those of 

 the inner nuclear layer and less oval than those of the outer nuclear layer (9 in the 

 figures). 



(10) A distinct optic fiber layer is not present and the optic nerve is nowhere 

 within the eye a compact strand of fibers. A loose flocculent strand of fibers passes 

 through the proximal part of the retina. Its path through the pigmented layer is 

 difficult to trace. Beyond the eye the optic nerve can be followed by means of the 

 fibrous sheaths and pigment cells associated with it rather than by the presence of 

 any fibers with a distinctly nervous structure. The optic nerve leaves the eye, not 

 at the proximal end or the narrow end of the pear, but anterior to the pigment 

 mass in the narrow part of the pear (fig. 19 b, n.op.}. 



