TYPHLICHTHYS SUBTERRANEUS. 121 



The eye is surrounded by a large mass of fat through which connective tissue 

 cells are scattered. A distinct separation of the orbital fat from the other fatty 

 tissues in this neighborhood by connective tissue membranes such as are found in 

 Amblyopsis is not noticeable in this species. A few pigment cells are found scattered 

 through the fat-mass. They are nowhere massed together so as to become evi- 

 dent to the naked eye. In one eye not a single pigment cell is found about its 

 surface, in another three are found on the surface of the connective tissue sur- 

 rounding the eye. In no case is the pigment about the eye of any significance, 

 for it is as abundantly found throughout the fatty tissue surrounding it. 



No trace of eye muscles are present. Scleral cartilages are entirely absent, a 

 condition in striking contrast to that found in Troglichthys rosce, with which this 

 species has been confounded. 



Sclera and Choroid. — The sclera and choroid coats are not separable in this 

 species. In specimens up to 40 mm. in length the eye is surrounded by a very 

 thin membrane containing here and there a nucleus, and in the region of the choroid 

 fissure and near the exit of the optic nerve a few capillaries. In the oldest speci- 

 men, 54 mm. long, the tissues about the eye are distinctly more fibrous, but even 

 here I have not been able to separate the layers. From the front of the eye a 

 strand of tissue similar to that surrounding the eye extends outward. A blood- 

 vessel reaches the eye with the optic nerve, and a few capillaries are found on the 

 surface of the eye and in the hyaloid membrane, but the details of their distribu- 

 tion I have not made out. This primitive condition of the outer layers of the eye 

 is not so striking as at first appears when the conditions in Chologaster are taken 

 into consideration, for even in Chologaster the choroid and sclera are insignificant. 



The Eyeball. — The eye is on an average 1.68 mm. in diameter and has reached 

 this size when the individual has reached 25 mm. in length. In specimens of this 

 length the cells of the retina are still undergoing division. In a specimen 20 mm. 

 long it has a diameter of 1.42 mm. Its maximum differentiation is not reached 

 at the time it first reaches its maximum diameter. The eye is probably potentially 

 functional throughout life as a light-perceiving organ. A minute vitreal cavity, 

 remnants of the hyaloid with its blood-vessels, outer and inner nuclear as well as 

 inner, and usually also the outer reticular layers are well differentiated, and the 

 optic nerve is certainly still connected with the brain at a time when the fish has 

 reached a length of 40 mm. 



The position of the eye is not fixed, so that in different series of sections, pre- 

 sumably cutting the head in the same planes, the choroid fissure occupies various 

 positions and the eyes are cut in various directions. With this general sketch the 

 various layers may be taken up in detail. 



Pigment Layer (1 in figs. 41 a, 43 c). — No pigment granules are present in the 

 eye, a condition in great contrast to that in either Amblyopsis or Chologaster, where 

 the pigment is least affected by the degeneration processes. The absence of 

 pigment in this eye is indeed unique among vertebrates. Whether pigment 

 is developed in earlier stages and disappears I have not been able to determine. 

 In the specimens 40 mm. and less in length the pigment layer consists of a series 

 of cells, but little separated from the underlying outer nucleated layer. The sepa- 

 ration between the layers is greatest near the exit of the nerve and at the iris. In 

 older individuals a considerable space is formed between the pigment layer and 



