62 BLIND VERTEBRATES AND THEIR EYES. 



THE FISHES. 



GENERAL REMARKS ON THE EYES OF FISHES. 



It is not the intention to review the literature on the normal eyes of fishes. A 

 list of papers dealing with their macroscopic aspect has been furnished by Ziegen- 

 hagen in 1895, while those dealing with minute structure have been enumerated 

 by Krause in 1886 and Cajal in 1894. The current literature is discussed periodi- 

 cally by Virchow in "Die Ergebnisse der Anatomie und Entwickelungsgeschichte." 



The topographical relationship of the cells of the retina obtained an entirely 

 new light by the application of the methylene-blue method chiefly on the part of 

 Dogiel, and the Golgi method principally through Ramon y Cajal. The layers 

 of the retina of fishes as made out by Ramon y Cajal are as follows, beginning at 

 the periphery and going toward the center of the eye: 



1. Epithelial-pigment layer. 5. Horizontal cells. 8. Inner molecular layer. 



2. Rods and cones. 6. Bipolar cells. 9. Ganglionic layer. 



3. Outer nuclear layer. 7. Spongioblasts. 10. Optic fiber layer. 



4. Outer molecular layer. 



Throughout this work the layers are designated on the figures and frequently in 

 the text by these numbers. The literature bearing on the eyes of the blind species 



will be given under the different species. 

 f~ ~\ ("*)( \ C3 C ) ^ e horizontal relations, especially the 



mosaic of the single and twin cones in the 

 retinas of fishes, has been dealt with by 

 Hanover, Miiller, Krause, Friis, Ryder, 

 V>^^s^ \^y vv Beer, Eigenmann, and Shafer. 



It was found that in many fishes the 

 single and twin cones form a regular mo- 

 saic. The number of parts entering into 

 each unit of the retinal mosaic is remark- 

 ably constant for any species, but differs 

 considerably in different species of fishes. 

 The "shape" of the unit differs in different 

 parts of the retina. The pattern may be 

 made up of twin cones only. 1 The axes or 

 lines joining the centers of the components 

 of each twin if continued may be at right 

 angles to each other and form a square 

 (fig. 22 a), or they may be approximately 

 parallel {Sebastodes, c), or they may be 

 fig. „ . Types of single and Double Cones'in Re.inas variously inclined to each other and form 

 of various Fishes. rhombs (Scorpama, b). 



In" other genera (Perca, Micropterus, Etheostoma, and Pimephales) a single 

 cone is placed in the center of each of the units of 4 twin cones (d). In still others 

 (Blennius, e) a single cone is added at each angle of the unit, and in still others 



1 Krause found only single cones in the eel. 



