THE EYES OF TYPHLICHTHYS. 123 



Cells of bizarre appearance were noted near the iris in one of the younger indi- 

 viduals (fig. 41 b). Some of these are long, club-shaped, with rounded end turned 

 inward, others the reverse, still others with long, elliptical outer segments and smaller 

 inner segments. 



The cones are certainly less developed than in Amblyopsis, while the reverse 

 is the case with the nuclei belonging to them. 



The outer nuclear layer seems but little more degenerate than in Chologaster as 

 far as differentiation is concerned, being of course very much more limited in extent. 



Outer Reticular Layer (4 in the figures). A distinct break between the outer 

 and inner nuclear layers, of varying thickness, where nuclei are absent or few 

 and far between, is present. A distinct boundary line for this layer does not exist, 

 and a reticulate appearance is only to be seen in short stretches, otherwise the 

 layer is only distinguished in the preparations by the absence of nuclei. In the 

 younger specimens examined this layer is not differentiated, the nuclear layers 

 forming one continuous structure. It is quite evident from this that tissue differen- 

 tiation is not completed in the eyes of this species till very late. 



The Inner Nuclear Layer. --The nuclei of the inner layer are of two sorts, larger, 

 granular, more faintly staining ones, and smaller, more homogeneous, deeper 

 staining ones. In one individual they are seen to be surrounded by a compara- 

 tively large cell body whose outlines are made distinct by the branches of the 

 Miillerian fibers. In thickness this layer exceeds both the nuclear layers in 

 Amblyopsis. It was not possible to identify nuclei belonging to the Mtillerian 

 fibers as such. Supporting fibers can be followed in some individuals from the 

 ganglionic layer through the inner reticular and the inner nuclear layers, in which 

 they branch to send processes between the regular cells (fig. 41 c). Once peculiar 

 horizontal nuclei were noticed on the inner face of this layer. They are marked 

 y in figure 41 a. 



The Inner Reticular Layer. Horizontal cells are not present in the inner reticu- 

 lar layer. Otherwise the layer offers no peculiarities. Owing to the persistence 

 of the union of the lips of the choroid fissure and the consequent merging of the 

 ganglionic into the outer layers at this point, the inner reticular layer appears 

 horseshoe-shaped in a vertical longitudinal section (fig. 41 a, 8). In a section going 

 through the plane of the choroid fissure (fig. 42 a, 8, and plate 3, fig. D, of Rhineura) 

 it appears as a central area in the eye, free from nuclei. This condition, which is 

 seen in all but the eyes of the oldest individuals, is of importance in interpreting 

 the conditions seen in Troglichthys roscc. In the older individuals the nuclear 

 layers become thin on either side of the choroid lips and the reticular layer ap- 

 proaches the pigment layer (fig. 41 c). The layer is well developed. Its relative 

 thickness may be gathered from the comparative table. 



The Ganglionic Layer. -- There is no distinct optic fiber layer. The ganglionic 

 layer consists of a single layer of cells irregularly disposed about the vitreal cavity 

 where this is present and forming a solid core of cells behind the vitreal region 

 inclosing blood-vessels and hyaloid nuclei. Some of the cells appear to send fibers 

 into the inner nuclear layer in the older retinas. These may be Miillerian nuclei, 

 since in Chologaster cornutus such are found in this layer. The total number of 

 nuclei counted in one example as belonging to this layer is 100, not very greatly 

 different from the number noticed in specimens of Amblyopsis. In specimens^up 



