134 BLIND VERTEBRATES AND THEIR EYES. 



central mass of the retina. No cells are developed in the reticular layer. The 

 optic fibers appear to pass directly through the reticular layer. This condition is 

 probably apparent rather than real. First the vitreous cavity disappeared, bring- 

 ing the ganglionic cells and the optic fiber layers together in the center of the 

 eye. This condition has just been reached by T. subterranean and Amblyopsis. 

 In the present species the ganglionic cells have disappeared from the center, and 

 only the optic fiber layer remains. This is represented by the individual fibers 

 passing from the ganglionic cells to the exit of the optic nerve. They do not form 

 a compact nerve, but the fibers pass individually to the exit in the most direct route 

 from their respective cells. 



I have been unable to trace the optic nerve for any distance beyond the eye. 

 In one case it leaves the eye as a loose bundle 12 /j. in diameter; in another case 

 it is more compact, being but 4 /a in diameter. It is surrounded by a sheath of 

 varying thickness and complexity. In one case there are a few cells about the nerve, 

 and these are covered by the tendon of the rectus muscle, which forms a complete 

 covering. 



Measurements in /a: The scleral cartilages vary from 18 to 40 in thickness. 

 The distance from the distal face of the retinal pigment to the ganglionic cells varies 

 from 30 to 40. The pigment cells have a maximum depth of 14, dwindling from 

 this to 2 or 3 on the sides. The nuclear layers reach a maximum thickness of but 

 10. The inner reticular layer, including the optic fiber layers, is about 40 in all 

 directions, reaching a proximo-distal length of 70. The lens measures from 10 to 15. 



AMBLYOPSIS SPEL^EUS. 



The eyes of Amblyopsis have been described by Tellkampf, Wyman, and Put- 

 nam. These authors gave general accounts of the eyes as far as this could be done 

 without serial sections, and their accounts are far from satisfactory. It is therefore 

 unfortunate that Kohl, who had less material of a supposed Typhlichthys from 

 Missouri, should have based a criticism of the facts observed by Wyman in Amblyop- 

 sis on what he saw, especially since scarcely a statement made by Kohl corresponds 

 to a condition found in Amblyopsis, or even the Typhlichthys sabterraneus from 

 Mammoth Cave. An abstract of Kohl's result are given under Troglichthys. 



Tellkampf first pointed out the presence of rudimentary eyes and states that 

 these can be seen in some specimens as black spots under the skin by means of 

 a powerful lens. The statement that the eyes are externally visible in some speci- 

 mens, which was afterwards thrown in doubt by Kohl, is perfectly correct. The 

 eye of Amblyopsis can be seen as a black spot with the unaided eye in specimens up 

 to 50 mm. in length. 



Wyman, in Putnam, figured the optic nerve, a lens, and muscular bands attached 

 to the exterior of the globe, but did not recognize them as homologues of the muscles 

 of the normal eyes of fishes. In a four-inch fish Wyman found the eye to be one- 

 sixteenth of an inch in its long diameter. A nerve filament was traced to the cranial 

 wall, but farther it could not be followed. The eye is made up of (1) a thin mem- 

 brane, the sclera ; (2) a layer of pigment cells, the choroid, which were most abundant 

 about the anterior part of the eye; (3) a single layer of colorless cells larger than 

 the pigment cells, the retina; (4) just in front of the globe, a lenticular-shaped 

 transparent body, the lens; (5) the whole surrounded by loose areolar tissue. 



