THE EYES OF TROGLICHTHYS. 



131 



where it ought to be most highly developed. Kohl has cut the Gordian knot by 

 excluding this pigment from the eye entirely by the choroid (sclera), but there is 

 certainly no such membrane intervening between this pigment and the rest of the 

 eye as Kohl has figured. On the contrary the choroid very clearly surrounds it, 

 and from its own epithelial structure there is no room for doubt as to its nature. 

 As said, its extension over the sides and back part of the eye differs materially in 

 different eyes. In a number of instances no pigment cells are present either on 

 the sides or at the proximal surface; in others the sides are well covered. If by 

 any means the tissues of the eye are separated from each other, the space is always 

 formed between the pigmented layer and the rest of the eye. Processes are at such 

 times seen to extend down from the pigment cells toward the rest of the retina. The 

 cell boundaries and nuclei of the pigment cells are for the most part distinct. The 

 cells are deepest over the distal pole of the eye and from this point they decrease 

 in size to the proximal pole. Toward the upper face, where the pigment epithelium 

 approaches the lens, the densely pigmented cells are transformed into much thinner 



</. c 



FIG. 47. Horizontal Section through only Eye with Central Ganglionic Cells. 

 From an Individual 34 mm. long. 



pigmentless cells. These are probably the homologues of the pigmentless cells 

 over the distal face of the eye of Amblyopsis, and, if so, are all that is left of the 

 outer layer of the iris. 



The explanation of the condition of the pigment epithelium in this eye presents 

 more difficulties than any other structure. In the eye of T. snbterraneus no pig- 

 ment is developed, but the pigment epithelium is normally developed. In this 

 eye pigment is formed in the cells that are present, but the epithelium has any- 

 thing but a normal structure. The pigment cells in the proximal face of the eye 

 have either disappeared or been displaced. The only other alternative, that they 

 are present but without pigment and indistinguishable from the cells of the outer 

 nuclear layer, while possible, is scarcely probable, for in many eyes there is but a 

 single layer of cells representing all of these structures, and in other cases even these 

 have vanished. The objection to the idea that the cells have vanished is to be 

 found in the fact that they are so well developed over the distal face. This point 

 can only be settled by a study of the development of the eye, but one other sug- 

 gestion may not be out of place. A comparison of this eye with that of Amblyopsis 



