THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 177 



the eye (Fig. 38) show the lips of the choroid fissure drawn apart with the contraction 

 of the retina, only the dorsal two-thirds of the eye reaching full development. From 

 a study of the embryos of this size the point of exit of the optic nerve which marks 

 the proximal end of the choroid slit alone gives evidence that potentially, at least, we 

 have to do with an eye from which a central cavity has disappeared. 



The optic nerve is well developed, arising apparently from the ventral cells of 

 the ganglionic mass, that is, those immediately lining the potential optic cavity. 



The pigment cells are well developed and have a varying depth in different parts 

 of the eye. They are low and without pigment over the front of the eye and the 

 ventral surface near the choroid slit. 



The retinal layers proper are differentiated into the ganglionic layer or mass 

 which occupies the central and lower part of the interior of the eye. Apparently 

 only the more centrally placed cells of this mass give rise to fibres. The inner reticu- 

 lar layer surrounds the ganglionic mass above and partly on the side, not at all below. 

 The nuclear layers are well developed, without a differentiation into outer and inner 

 layers or any indication of an outer reticular layer. The latter structure is apparently 

 never formed at all. 



5. Seven-millimetre Stages. — The variability in the rate of development of the 

 eye is well seen in a series of specimens about 7 millimetres long and whose eyes are 

 little if any beyond the stage of development reached in other specimens only 5 milli- 

 metres long taken from another female. In the former the eye is in contact with the 

 dura proximally, but is withdrawn from the epidermis by 36 micra or more. A strand 

 of cells extends from the eye upward and outward to the thinnest part of the epider- 

 mis. The epidermis is distinctly thinner over the eye than in neighboring regions. 



The eyeball is subspherical with a shallow groove along its ventral surface repre- 

 senting the choroid slit (Fig. 40). 



In half of the specimens of this size examined no lens could be detected. In 

 one the lens was a comparatively large pear-shaped structure whose cells were under- 

 going degeneration, if the numerous dark granules in them were indicative of degen- 

 eration. In one individual in which no lens could be found on one side, a small group 

 of cells lying between the eye and the skin of the other side was probably the lens. 

 The cells were breaking apart and the outline of the structure as a whole was irregular. 

 In all cases the lens lies outside the iris, and in fact the entire vitreous space is not 

 large enough to hold the lens in such eyes as still show this structure. 



The pigment layer is pigmented over the dorsal part of the eye. In vertical 

 sections no pigment appears below the entrance of the optic nerve. The iridian part 

 of the layer is, as usual, without pigment. The ganglionic cells, as in the last stages 



