SUMMARIAL ACCOUNT OF THE EYE OF THE AMBLYOPSID/E. 



179 



gaster cornutus, developed in spots in Typhlichthys, and no longer distinguishable 

 in the other species. 



(h) The layers of horizontal cells are represented in papilliferus by occasional 

 cells ; they are rarer in cornutus and beyond these have not been determined with 

 certainty. 



(i) The inner nuclear layer of bipolar and spongioblast^ cells is well developed 

 in C. papilliferus and C. agassizii. In cornutus it is better developed in the young 

 than in the older stages, where it forms but a single layer of cells. There is evi- 

 dently in this species an ontogenetic simplification. In the remaining species it is, 

 as mentioned above, merged with the other nuclear layer into one layer which is 

 occasionally absent in Troglichthys. 



(j) The inner reticular layer is relatively better developed than any of the other 

 layers, and the conclusion naturally forces itself upon one that it must contain 

 other elements besides fibers of the bipolar and ganglionic cells, for, in Amblyopsis 

 and Troglichthys, where the latter are very limited 

 or absent, this layer is still well developed. Hori- 

 zontal cells have only been found in the species of 

 Chologaster. 



(k) In the ganglionic layer we find again a com- 

 plete series of steps from the most perfect eye to the 

 condition found in Troglichthys. In papilliferus 

 and agassizii the cells form a complete layer one 

 cell deep except where they have given way to the 

 optic fiber tracts which pass in among the cells 

 instead of over them. In cornutus the cells have 

 been so reduced in number that they are widely 

 separated from each other. With the loss of the 

 vitreous cavity the cells have been brought together 

 again into a continuous layer in Typhlichthys, 

 although there are much fewer cells than in cornutus 

 even. The next step is the formation of a solid core 

 of ganglionic cells, and the final step the elimination of this central core in Troglich- 

 thys, leaving but a few cells over the anterior face of the retina. 



(/) Mullerian nuclei are found in all but Amblyopsis and Troglichthys. In 

 C. cornutus they lie in part in the inner reticular and the ganglionic layer. Cells of 

 this sort are probably also found among the ganglionic cells of Typhlichthys. 



We thus see that the simplification or reduction in the eye is not a horizontal 

 process. The purely supporting structures like the scleral cartilages 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 thickness 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 

 degree of development is evidently associated with the degree of development of 

 the pigmented layer. As long as the cones are developed, the pigmented layer is 

 well developed, or vice versa. 



Fig. 67. Diagram showing per cent'of Total Thick- 

 ness of each Layer of Retina in i, Zygonectes 

 nototus; 2 and 3, Chologaster cornutus, 27 mm. 

 long and 43 mm. long; 4, Chologaster papilli- 

 ferus, 20 to 39 mm. long, and 5, 55 mm. long; 

 6, Chologaster agassizii, 38 mm. long and 7.62 

 mm. long; 8, Amblyopsis; 9. Typhlichthys; 

 10, Troglichthys. 



