THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 179 



In ten specimens sectioned, all of them from 9.5 to 10 millimetres 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 three) cells are pigmented. 

 The maximum thickness of this layer is 12 micra. 



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

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

 otherwise be distinguished. 



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

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

 pigmented layers along the ventral line. 



The outer and inner nuclear layers were represented by about four rows of nuclei 

 immediately within the pigmented layer. The cells represented by these nuclei were 

 not separable into an outer and an inner layer histologically, nor was there any break 

 indicating the presence of any outer reticular layer. The cells formed a compact 

 layer of approximately uniform thickness. There were no indications of cones in 

 any of the eyes examined. 



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

 roid fissure, where, as has been said above, the nucleated layers of the retina met- 

 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 (PL XV ; Fig. 46). 



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

 ganglionic cells. This mass was 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 were continuous with those of the nuclear layers. In 

 the distal face of the trumpet, in what would be its hollow end, there was a distinct 

 conical area free from cells and abundantly supplied with fibres (Fig. 47). It is post 

 sible that this represents the optic fibre layer. The optic nerve was well developed, 

 but its fibres seemed to go to their respective cells directly without first going to this 

 apparent optic fibre layer. 



The outer nuclear layers measured about 20 micra, the inner reticular about 8 

 micra, and the ganglionic layer about 32 micra in thickness. 



No specimens between 10 and 25 millimetres have been captured. The changes 

 taking place during this period are insignificant. (Compare Figs. 4, 5.) 



