DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 165 



In order to compare the ratio between the ontogenetic and phylogenetic rates 

 of degeneration, it is necessary to use some stage in the development of the eye as 

 the point which phylogenetic degeneration has reached. For such a point we 

 shall use the optimum reached by various parts of the eye during their develop- 

 ment. It is certain that the phylogenetic stage is below this optimum, that some 

 of the degeneration in individual eyes is due to phylogeny, but since we do not 

 know how much of the descent from the optimum is due to heredity and how 

 much to the peculiarities of the environment and the resulting functionless life of 

 the parts during the life of the individual, it will be best to take the optimum as 

 above indicated. 



All phylogenetic time is taken as a unit, although some parts of the eye have 

 been degenerating longer than others. The ontogenetic degeneration leads from 

 the optimum to the vanishing point for most parts of the eye. 



Ontogenetically the lens degenerates very rapidly, reaching its vanishing point 

 from its optimum during the period in which the fish grows not more than 5 mm. in 

 length. The rate of its phylogenetic degeneration must have been proportionately 

 rapid, for at its optimum in Amblyopsis it is minute and its cells are undifferentiated. 

 In the epigean relatives of Amblyopsis the lens is one of the parts least affected, so 

 that it must have degenerated very rapidly in its later phylogenetic history, after 

 the fish had entered the caves. 



At its best the vitreous body is so inappreciable in amount that I have not been 

 able to consider its ontogenetic degeneration. Its phylogeny has approached the 

 vanishing point toward which most parts of the eye are heading. 



The retina may be considered in its extent and in the degree of the histogenic 

 differentiation of its parts. In the matter of its extent or size there is little change 

 from its optimum until its disintegration in old age. Its ontogenetic changes are 

 slight. Its optimum is comparable with that of the lens and indicates a rapid and 

 great reduction from the lowest retina of epigean relatives. The ontogenetic and 

 phylogenetic rates of degeneration in the extent of the retina differ greatly, the 

 former having come practically to a standstill. 



In its histogenic differentiation the retina is not comparable with the lens, for it 

 rises above the embryonic phases. In fact, in its histogenic differentiation the retina 

 rises far above the requirements of the case, and the most highly developed eye of 

 Amblyopsis approaches the lowest of its epigean relatives. Over any given area it 

 is doubtful whether the ganglionic and inner reticular layers are more degenerate or 

 as degenerate as the same parts in the eyes of Chologaster cornuhis. It is certain 

 that in their highest development the parts between the inner reticular and the pig- 

 mented layers are below the lowest point reached by the corresponding parts in the 

 epigean species mentioned. The same is true of the pigmented epithelium. 



The simplification of the structure of the retina from its maximum to its mini- 

 mum in ontogeny is of greater extent than its simplification from the lowest differ- 

 entiated retina found in epigean species to the maximum found in Amblyopsis. 



From the foregoing we may conclude that there is no constant ratio between 

 the extent and degree of ontogenetic and phylogenetic degeneration, and that the 

 observed rate of ontogenetic degeneration is not necessarily proportionate to the 

 rate of phylogenetic degeneration inferred from the degree of degeneration of the eye 

 at its optimum. 



