Makch 30, 1900.] 



SCIENCE. 



^97 



layers have gone a step further in their de- 

 generation than in cornutus, but the great- 

 est modification has taken place in the 

 dioptric arrangements. 



In Troglichthys even the mass of ganglionic 

 cells present in the center of the eye as the 

 result of the collapsing after the removal 

 of the vitreous body has vanished. The 

 pigmented epithelium, and, in fact, all the 

 other layers, are represented by mere frag- 

 ments. 



The eye of lyphliehthys has degenerated 

 along a different line. Tliere is an almost 

 total loss of the lens and vitreous body in 

 an eye like that of papiWferus without an 

 intervening stage like that of cornutus, and 

 the pigment layer has lost its pigment, 

 whereas in Ambhjopsis it was retained. 



The salamanders bridge the gap existing 

 between the Chologasters and the blind mem- 

 bers of the Amblyopsidse. But even at the 

 risk of monotonous-repetition I want again 

 to call attention to the fact that the sala- 

 manders do not belong to tlie same series 

 as the Amblyopsidse. The dioptric arrange- 

 ments of Tijphlotriton are all normal, the 

 retina is normal in the young, but the rods 

 and cones all disappear with the change 

 from the larval to the adult condition. In 

 Tijphlomolge, the lens and largely the vitre- 

 ous body are gone and the eye has collapsed. 

 The vitreous body is, however, much better 

 represented than in the blind Amblyopsidse 

 and the iris is, especially in the young, 

 much better developed than in the fishes. 



2. The retina. 



(a) There is more variety in the degree 

 of development of the pigment epithelium 

 than in any other structure of the eye. 

 Hitter has found that in Tijphlogohius this 

 "layer has actually increased in thickness 

 concomitantly with the retardation in the 

 development of the eye, or it is quite pos- 

 sible with the degeneration of this par- 

 ticular part of it. An increase of pigment 

 is an incident to the gradual diminution in 



functional importance and structural com- 

 pleteness." There is so much variation in 

 the thickness of this layer in various fishes 

 that not much stress can be laid on the ab- 

 solute or relative thickness of the pigment 

 in any one species as an index of degenera- 

 tion. 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 actually not 

 much, if any, thicker than that of Zygonedes ; 

 exception must be made for specimens of 

 the extreme size in pap)llUferus and agas- 

 sizii. In other words, primarily the pig- 

 ment layer has retained its normal condi- 

 tion while the rest of the retina has been 

 simplified and there may eyen be an in- 

 crease in the thickness of the layer as one 

 of its outogenic 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 Typhlich- 

 thys, which is undoubtedly derived from a 

 Chologaster-Vike ancestor, no pigment is de- 

 veloped. The layer retains its epithelial 

 nature and remains apparently in its em- 

 bryonic condition. It may be well to call 

 attention here to the fact that the cones are 

 very sparingly developed, if at all, in this 

 species. In Amblyopsis, in which the de- 

 generation of the retina has gone further 

 but in which the cones are still well de- 

 veloped, 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 jjart of the eye. We thus find a de- 

 velopment of pigment with an imperfect 

 layer in one case (Troglichthys) and a full 

 developed layer without j)igment in another 

 {Typhlichthys) . In the Chologasters the pig- 

 ment is in part prismatic ; in the other spe- 

 cies granular. 



