498 



SCIENCE. 



[N. S. Vol. XI. No. 274. 



(i) In the outer nuclear layer a com- 

 plete series of steps is observable from the 

 two-layered condition in papilliferus to the 

 one-layered in cornutus, to the undefined 

 layer in Typhlichthy.s and the merging of the 

 nuclear in Amhlyopsis, and their occasional 

 total absence in TroglieJithys. The rods dis- 

 appear first, the cones long before their 

 nuclei. 



Diagram showing tlie per cents, of the total thick- 

 ness of each of the layers of the retina in 1. Zij- 

 ■^onecies notaius. 2. Chohigaster cornutus 27mm. long 

 3. 43 mm. long. i. Chologastn- papill if eras 29-39 mm 

 and 5., 55 mm. long. 6. Cholognslir agassizn 38 mm 

 long and 7., 62 mm. long. 8. Amhiijnpsis. 9. Typhlich- 

 thys. 10. Ti-ogUchlliys. 



(c) The outer reticular layer naturally 

 meets with the same fate as the outer 

 nuclear layer. It is well developed in 

 papilliferus, evident in C. cornutus, developed 

 in spots in Typhliohthys, and no longer dis- 

 tinguishable in other species. 



(cQ The layers of horizontal cells are 

 represented in papilUJenis by occasional 

 cells ; they are rarer in cornutus and beyond 

 these have not been detected. 



(e) The inner nuclear layer of bipolar 

 and spongioblastic cells is well developed 

 in papilliferus. 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 evidently in this species an 



ontogenic simplification. In the remaining 

 species it is, as mentioned above, merged 

 with the outer nuclear layer into one layer 

 which is occasionally absent in TroglieJithys. 



(f) 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. Horizontal cells in this layer 

 have only been found in the species of Cholo- 

 y aster . 



(g) In the ganglionic layer we find again 

 a complete series of steps from the most per- 

 fect eye to the condition found in Trog- 

 lieJithys. In papilliferus the cells form a com- 

 plete layer one cell deep except where the 

 cells 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 TypJilicJitJiys, 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 Trog- 

 licJitJiys, leaving but a few cells over the 

 anterior face of the retina. 



(/i) Miillerian nuclei are found in all but 

 TroglieJitJiys. 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 TypJi- 

 licJithys. 



3. The optic nerve shows a clear grada- 

 tion from one end of the series of fishes 

 to the other. In CJiologaster piapilliferus it 

 reaches its maximum development. In cor- 

 nutus which possesses an eye larger than 

 2Ktpilliferus, but in which the ganglionic 



