THE EYES OF TROGLICHTHYS. 133 



The Retina : The elements of the retina proper, i.e. the ganglionic, nuclear, and 

 reticular layers, form a vesicle arranged so that the cellular elements surround a 

 central (the inner) reticular layer. These may be taken up seriatim. The cellular 

 elements are of three sorts. 



(i) Behind the lens and behind the pigment layer, sometimes also over the side 

 of the retina, lie a few cells with elongated nuclei (fd,l, in figs. 45 a, b, 46 b, 49 a) 

 and so arranged as to suggest an epithelial covering for the underlying structures. 

 Some of these cells were supposed by Kohl to represent the choroid, with which they 

 have absolutely no connection. It is possible that some of these lateral cells are 

 modified pigment cells, but even this seems doubtful. I am unable to refer the 

 cells of this nature situated laterally over the retina to any structure in the normal 

 retina. Such cells are, however, found in the eyes of T. subterraneus between 

 the pigment epithelium and the nuclear layers (fig. 41 a), and whatever their origin 

 the two structures are unquestionably homologous in the two eyes. It is probable 

 that the cells with elongated nuclei to be found behind the lens are of different 

 origin and significance. They may be the remains of the elongated cells found 

 in the inner surface of the iris of Chologaster, cells which are still present in both 

 Amblyopsis and T. subterraneus. It is also possible that they are the remains of 

 the hyaloid nuclei. 



(2) The ganglionic cells, which in Typhlichthys are arranged around the vestige 

 of the vitreal cavity and in Amblyopsis form a central core and are distributed over 

 the front of the retina, are in this species practically confined to the latter location. 

 All there is left of the central core of ganglionic cells in Troglichthys rosce is three 

 cells in the most highly developed eye found (fig. 47 and plate 10, fig. 11). In the 

 other eyes no indication of these cells was detected. If these cells come to be formed 

 at all in the present eye, they migrate forward, where they form the anterior wall of 

 cells surrounding the inner reticular layer. The fibers of the ganglionic cells extend 

 directly from the ganglionic cells through the reticular layer to the exit of the optic 

 nerve. The cells must, as Kohl has suggested, have undergone a rotation on their 

 axes to send their fibers directly to the optic nerve, unless only the lineal descendants 

 of those ganglionic cells immediately surrounding the entrance of the optic nerve 

 in Chologaster are here represented, a supposition not to be entertained. The 

 ganglionic nuclei are occasionally notably larger than the nuclei of the rest of the 

 retina, but they are by no means always so. 



(3) The cells of the nuclear layers join those of the ganglionic layer. The 

 cells of the inner and outer nuclear layer and the horizontal cells are indistinguish- 

 able from each other. They form, in the most highly developed condition, figure 47, 

 3-7, a layer three cells deep covering the sides and the proximal surface of the 

 inner reticular layer. In some cases the layer is reduced to a single series of cells, 

 and even these are occasionally absent. There is no sharp distinction between the 

 nuclei of this layer and those of the ganglionic layer, so that the boundary between 

 these cells and the ganglionic cells is not marked. In some instances these cells 

 appear to be directly continuous with the cells surrounding the origin in the optic 

 nerve. This condition led Kohl to imagine that the primary optic stalk had 

 become filled with nerve fibrils. 



Of the reticular layers the outer (8, in fig. 47) is not developed. The inner 

 reticular layer forms, with the optic fibers traversing it, the spherical or pear-shaped 



