138 



BLIND VERTEBRATES AND THEIR EYES. 



is not simply a miniature normal eye. The whole eye has collapsed with the dis- 

 appearance of the vitreous humor, and looked at in this light there is no difficulty 

 in the position of the cartilages which have fallen together over the front of the eye. 

 The presence of granular nuclei in front of the eye over the region of the iris has 

 been noted by Kohl in Troglichthys and observed by me. These nuclei are probably 

 the homologues of the nuclei found in the ligamentum pectinatum of Chologaster. 

 In shape, number, and size the scleral cartilages differ very much. In one 

 instance cartilages extend continuously from the exit of the optic nerve more than 

 half-way over the side and around the front of the eye. In another a single cartilage 

 lies directly in front of the eye, and on the opposite side of the same individual 

 a single cartilage lies behind the eye. The sclera is much more developed than in 

 Chologaster, consisting, aside from the cartilages, of an abundant fibrous tissue. 



Fig. 50. Section through the Eye of Amblyopsis spHaus 75 rum. long, killed with Chromic Acid and stained 

 with Biondi-Ehrlich's three-color mixture. This is the most highly developed eye seen, 2 mm. and 4. 



The choroid is a thin membrane closely applied to the eye. It contains a few 

 oval nuclei parallel with the surface of the eye. Pigment cells are few, irregularly 

 scattered, and not at all uniform in different eyes. The pigment cells are rounded 

 masses usually much thicker than the whole of the choroid in regions devoid of 

 pigment. About the entrance of the optic nerve is frequently a large accumulation 

 of pigment corresponding with the increase in the amount of choroidal pigment 

 in Chologaster at the same place. Even this mass is not uniformly present. Some- 

 times granular masses interspersed with pigment are found here, which give one 

 the impression of a degenerating mass. An especially large accumulation of pig- 

 ment is found in the eye represented by figure 53. Blood-vessels are present in the 

 choroid. They are apparently as great in relative capacity as in Chologaster. In 

 an individual with the vascular system injected, a vessel, 0.01 mm. in diameter, 

 approaches the eye with the optic nerve, but it does not enter the ball with the latter. 



