41 
development. In cornutus, which possesses an eye larger than papillif- 
erus, but in which the ganglionic layer is simplified, the nerve is measur- 
ably thinner. In Typhlichthys the nerve can be traced to the brain even 
in specimens 40 mm. long; i. e., in specimens which are evidently adult. 
In Amblyopsis the nerve can be followed to the brain in specimens 25 
mm. long, but in the adult I have never been able to follow it to the brain. 
In Troglichthys it has become so intangible that I have not been able to 
trace it for any distance beyond the eye. 
We thus see that the simplification or reduction in the eye is not a 
horizontal process. The purely supporting structure, like the scleral car- 
tilages have been retained out of all proportion to the rest of the eye. The 
pigment layer has been both quantitatively and qualitatively differently 
affected in different species. There was primarily an increase in the thick- 
ness of this layer, and later a tendency to total loss of pigment. The 
degeneration has been more uniformly progressive in all the layers within 
the pigment layer. The only possible exception being the inner reticular 
layer which probably owes its retention more to its supporting than to its 
nervous elements. Another exception is found in the cones, but their de- 
gree of development is evidently associated with the degree of develop- 
ment of the pigmented layer. As long as the cones are developed the pig- 
mented layer is well developed or vice versa. 
We find, in general, that the reduction in size from the normal fish 
eye went hand in hand with the simplification of the retina. There was at 
first chiefly a reduction in the number of many times duplicated parts. 
Even after the condition in Chologaster papilliferus was reached the de- 
generation in the histological condition of the elements did not keep pace 
with the reduction in number (vide the eye of cornutus). The dioptric 
apparatus disappeared rather suddenly and the eye as a consequence col- 
lapsed with equal suddenness in those members which, long ago, took up 
their abode in total darkness. The eye not only collapsed, but the number 
of elements decreased very much. The reduction was in the horizontally 
repeated elements. The vertical complexity, on which the function of the 
retina really depends, was not greatly modified at first. 
In those species which took up their abode in total darkness the de- 
generation in the dioptric apparatus was out of proportion to the degen- 
eration of the retina, while in those remaining above ground the retinal 
structures degenerated out of proportion to the changes in the dioptric 
apparatus, which, according to this view, degenerates only under condi- 
