MUSEUM OF COMPARATIVE ZOOLOGY. 97 



ing pigment ; in the incomplete differentiation of the layers of the retina, 

 there being in some imlividnals scarcely more than a trace of the external 

 reticular layer separating the two nuclear layers, and there being in no 

 specimen studied a retina sufficiently developed to enable one to homol- 

 ogize with certainty tlie layers marked out ; in the minute size of the 

 optic nerve, and the fact that it is ensheathed in a thick layer of pig- 

 ment for nearly its entire 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 arc at all 

 pigmented in the normal eye ; and, secondly, in the presence of pigment 

 in regions where none is found in the normal eye, as in the hyaloid 

 membrane (Plate II. Fig. 12, jng."). 



No undoubted instances of degenaration 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, tiiough 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 vertebrates 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 degenei'ative 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 Myxinida?, 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 on- 

 togeny follow the reverse order of their phylogeny. The little that may 

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

 tive character. This is found in the breaking down and resorption of the 

 lens, — habitually in Proteus, and probably occasionally in Typhlogo- 

 bius, — possibly in the excess of pigment in the iris and pigment layer 

 of the retina, and particularly in its occasional presence in the hyaloid 

 membrane of the Typhlogobius eye, while no evidence of actual degen- 

 eration in the retina appears in connection with these. The possible case 

 of a degenerating retina in Typhlogobius is neglected in this considera- 

 tion, since, as pointed out, the lens being absent in the same eye, it is 

 immaterial whether it be considered or not. 



VOL. XXIV. — NO '■]. 7 



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