154 BLIND VERTEBRATES AND THEIR EYES. 



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, i. e., in which it does not develop. 



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 fibers. The inner 

 reticular 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. 



SEVEN-MILLIMETER STAGES. 



The variability in the rate of development of the eye is well seen in a series of 

 specimens about 7 mm. long and whose eyes are little if any beyond the stage of 

 development reached in other specimens only 5 mm. 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 In the former the eye is in contact with the dura proximally, but is withdrawn from 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 jx or more. A strand of cells extends from the eye upward 

 and outward to the thinnest part of the epidermis. The epidermis is distinctly 

 thinner over the eye than in neighboring regions. 



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

 representing the choroid slit (fig. 62 e). 



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 

 undergoing degeneration, if the numerous dark granules in them were indicative 

 of degeneration. In one individual in which no lens could be found on one side, 

 that of the other side was probably represented by a small group of cells lying 

 between the eye and the skin'(/;w. 63 c). The cells were breaking apart and the 

 outline of the structure as a^whole was irregular. In all cases the lens lies out- 

 side 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 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 fibers to the optic nerve. Fibers 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 ju. 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 



