178 BLIND VERTEBRATES AND THEIR EYES. 



comes, the lens, both phylogenetically and ontogenetically, disappears rapidly. In 

 Typhlogobius Ritter found the lens absent in one very old individual, and Cope 

 found that in Gronias the lens is sometimes present on one side, while not on the 

 other. In Amblyopsis and Typhlichthys it has degenerated to a mere vestige, or 

 is gone altogether. Ritter, after considering the structure of degenerate eyes as 

 far as known at the time, came to the conclusion "that the lens disappears before 

 the retina; and that, where degeneration takes place at all in ontogeny, the lens is 

 affected first and most profoundly." With the first part of this statement the more 

 recent observations are in full accord. It is, however, doubtful whether the lens 

 is ever the first part affected ; in fact the retina always leads, but certainly the lens, 

 if affected at all, is affected profoundly. 



(f) There is more variety in the degree of development of the pigment epithelium 

 than in any other structure of the eye. Ritter has found that in Typhlogobius 

 this "layer has actually increased in thickness concomitantly with the retardation 

 in the development of the eye, or it is quite possible with the degeneration of this 

 particular part of it. An increase of pigment is an incident to the gradual diminu- 

 tion in functional importance and structural completeness." There is so much 

 variation in the thickness of this layer in various fishes that not much stress can be 

 laid on the absolute or relative thickness of the pigment in any one species as an 

 index of degeneration. While the pigment layer is, relative to the rest of the retina, 

 very thick in the species of Chologaster, it is found that the pigment layer 

 of Chologaster is not much if any thicker than that of Zygonectes, but exception 

 must be made for specimens of the extreme size in papilliferus and agassizii. 

 In other words, primarily the pigment layer has retained its normal condition, 

 while the rest of the retina has been simplified, and there may even be an increase 

 in the thickness of the layer as one of its ontogenetic modifications. Whether 

 the greater thickness of the pigment in the old Chologaster is due to degeneration 

 or the greater length of the cones in a twilight species I am unable to say. 



In Typhlichthys, which is undoubtedly derived from a Chologaster-like an- 

 cestor, no pigment is developed, the layer retains its epithelial nature and remains 

 apparently in its embryonic condition. It may be well to call attention here to the 

 fad that the cones are very sparingly developed, if at all, in this species. In 

 Amblyopsis, in which the degeneration of the retina has gone farther, but in which 

 the tones are still well developed, the pigment layer is very highly developed, but 

 not by any means uniformly so in different individuals. The pigment layer reaches 

 its greatest point of reduction in rosce where pigment is still developed, but the layer 

 is fragmentary except over the distal part of the eye. We thus find a development 

 of pigment with an imperfect layer in one case, Troglichthys, and a full-developed 

 layer without pigment in another, Typhlichthys. In the chologasters the pigment 

 is prismatic; in the other species granular. 



(/) In the outer nuclear layer a complete series of steps is observable from the 

 two layered condition in papilliferus to the one-layered in cornutus,lo the undefined 

 layer in Typhlichthys and the merging of the nuclear layers in Amblyopsis, and 

 their occasional total absence in rosce. The single cones disappear first, the cones 

 long before their nuclei. 



(.(,') The outer reticular layer naturally meets with the same fate as the outer 

 nuclear layer. It is well developed in papilliferus and agassizii, evident in Cholo- 



