Ixx Journal of Comparative Neurology. 



to a quite limited extent, the rest of that layer being composed ex- 

 clusively of pigment ; in the fact that the choroid gland is composed 

 entirely of pigment ; in the fact that the iris, though of fully the nor- 

 mal thickness, is almost devoid of pigment, there being on its outer 

 surface in some specimens a small amount of cellular material, which 

 probably represents the ligamentum annulare ; in the great propor- 

 tional thickness of the pigment layer of the retina and the entire ab- 

 sence in it of anything excepting pigment ; in the incomplete differ- 

 entiation of the layers of the retina, there being in some individuals 

 scarcely more than a trace of the external reticular layer separating 

 the two nuclear layers, and there being in no specimen studied a ret- 

 ina sufficiently developed to enable one to homologize with certainty 

 the layers marked out ; in the minute size of the optic nerve, and the 

 fact that it is ensheathed in a thick layer of pigment for nearly its en- 

 tire course through the retina ; and, finally, in the small size of the 

 motores oculi. 



6. The surest evidences of actual degeneration are found, first, 

 in the greatly augmented quantity of pigment in all the portions that 

 are at all pigmented in the normal eye ; and, secondly, in the pres- 

 ence of pigment in regions where none is found in the normal eye, as 

 in the hyaloid membrane. 



No undoubted instances of degeneration through the breaking 

 down and dissolution of the tissue without the formation of pigment, 

 such as have been described particularly by Looss, have been found, 

 though in a single specimen (the one in which no lens is present) a 

 process of this nature may be taking place. 



7. On comparing the eyes of all blind vetebrates that have been 

 most carefully studied, we find that, in the several degrees of incom- 

 pleteness of development represented by the different species, all may, 

 in a general way, be said to be passing along the same degenerative 

 road. There are apparently, however, a few interesting exceptions to 

 this. The most marked of these exceptions is found in the entire 

 lack of pigment in the eyes of the Myxinidae, whereas in all other 

 rudimentary eyes an increase of this substance over what exists in 

 normal eyes is found. 



8. The eyes of blind vertebrates furnish very little evidence on 

 the question whether structures in undergoing actual degeneration in 

 ontogeny follow the reverse order of their phylogeny. The little that 

 may be regarded as bearing on this point is without much doubt of an 

 affirmative character. This is found in the breaking down and re- 

 sorption of the lens, — habitually in Proteus, and probably occasion- 



