156 BLIND VERTEBRATES AND THEIR EYES. 



shallow depression in the distal face of the eye with a very narrow slit, sometimes 

 a line, separating the iris from the solid mass of cells representing the retina. The 

 vitreous cavity formed by the ventral invagination, that is, proximal of the iris, is 

 obliterated in some individuals except in so far as the absence of pigment along a 

 median line and in the union of the ganglionic layer with the pigmented layer along 

 this line indicates its presence. The choroid fissure has been noted in an individual 

 over ioo mm. long, so that evidently in some cases it may not close. Blood-vessels 

 are still present in the vitreous cavity as far as it is developed. The distance from 

 the exit of the optic nerve to the ventral margin of the pupil is considerably less 

 than the distance between the exit of the optic nerve and the dorsal margin of the 

 pupil. 



A few nuclei, probably the remnants of the hyaloid membrane, lie over the 

 distal face of the retina. 



In 10 specimens sectioned, all of them from 9.5 to 10 mm. long, the lens has 

 disappeared without leaving any trace. 



The pigmented layer increases in thickness from the iris to the exit of the optic 

 nerve. Its pigmentation also increases from the iris to the optic nerve. Within 

 any one cell the pigment is uniformly distributed. In the dorsal part of the eye the 

 pigment reaches to the iris, while in the ventral it does not reach so far, and in fact 

 in a line from the optic nerve to the iris very few (only about 3) cells are pigmented. 

 The maximum thickness of this layer is 12 /a. 



The inner cells of the iris have taken on their elongate shape which distin- 

 guishes them in the eye of the adult, where the region of the iris and pupil can not 

 otherwise be distinguished. 



The layers of the retina are now well developed except that the ganglionic mass 

 of cells occupying the center of the eye is continuous with the outer nuclear and the 

 pigmented layers along the ventral line. The outer and inner nuclear layers are 

 represented by about 4 rows of nuclei immediately within the pigmented layer. 

 The cells represented by these nuclei are not separable into an outer and an inner 

 layer histologically, nor is there any break indicating the presence of any outer 

 reticular layer. The cells form a compact layer of approximately uniform 

 thickness. There are no indications of cones in any of the eyes examined. 



The inner reticular layer is well developed except along the region of the cho- 

 roid fissure, where, as has been said above, the nucleated layers of the retina meet. 

 There is possibly one exception to this in one of the eyes, in which the reticular 

 layer surrounded the optic nerve at its entrance to the eye (fig. 63/). 



The space ventral to the central axis of the eye is occupied by the mass of 

 ganglionic cells. This mass is irregularly trumpet-shaped, with the narrow end 

 of the trumpet at the entrance of the optic nerve and the wide end at the distal 

 part of the retina, where its cells are continuous with those of the nuclear layers. 

 In the distal face of the trumpet, in what would be its hollow end, there is a dis- 

 tinct conical area free from cells and abundantly supplied with fibers (fig. 63 g). 

 It is possible that this represents the optic-fiber layer. The optic nerve is well 

 developed, but its fibers seem to go to their respective cells directly without first 

 going to this apparent optic-fiber layer. The outer nuclear layers measure about 

 20 /a, the inner reticular about 8 fi, and the ganglionic layer about 32 ti in thickness. 

 The changes taking place between 10 and 25 mm. are insignificant. 



