178 THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA . 



described, are exposed to the exterior through the choroid fissure, or where this is not 

 evident there is no differentiation into different layers along the line of the choroid 

 fissure. The ganglionic cells placed at the distal face of the eye give off fibres to the 

 optic nerve. Fibres have not been definitely traced to the cells of the same series 

 occupying the proximal or middle position. The optic nerve reaches a thickness of 

 20 micra and breaks up into bundles a short distance within the eye. These bundles 

 radiate, forming an incomplete funnel-shaped structure. The incomplete inner 

 reticular layer only partially separates the ganglionic and the nuclear layers. The 

 relative development of the pigment layer and the inner reticular layer both show a 

 less degree of differentiation than the same layers in the eyes of another series of 

 larvae only 6 millimetres long. This is due to the individual variation in the rate of 

 development, not to degeneration since the last stage. 



Dividing cells are found in the nucleated layer. 



In the nuclear layers some nuclei elongated in a vertical direction are probably 

 the nuclei of the Mullerian fibres. 



6. Nine- to Ten-millimetre Stages — In larvse 9 to 10 millimetres long the eyes 

 lie from 60 to 100 micra removed from the epidermis and in contact with the brain- 

 capsule or but little separated from it. Their average measurements are: longi- 

 tudinal diameter 114 micra, antero-posterior 98 micra, vertical 106 micra (Plate XV, 

 Figs. 45-49). 



The epidermis over the eye has assumed the thickness found over neighboring 

 regions, and from now on till death by old age there are no external modifications to 

 indicate the former position of the cornea. 



The pupil is still open, and also the choroid fissure in the region of the pupil (PL 

 XV, Figs. 45, 47). In the proximal parts the choroid fissure is indicated by the 

 absence of pigment along the ventral line (Fig. 46). The vitreous cavity is a shal- 

 low 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 indi- 

 cates its presence. The choroid fissure has been noted in an individual over 100 milli- 

 metres 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, he over the distal face of theretina. 



