﻿40 



BLIND VERTEBRATES AND THEIR EYES. 



demonstrable structure. Just at the margin of the place where the pigment has 

 been torn from the retina one of these is drawn out to a great length. The j)igment 

 in this indi\idual extends in places down between the nuclei of the cones. This 

 latter condition appears in a very exaggerated form in the eye of Typhlomolgc. 

 In tangential section this condition and the filmy rods give rise to the appearance 

 represented in figure lo ^s,'. 



Distinct signs of ontogenetic degeneration are also seen in other parts of the 

 retina. For instance, many nuclei of the inner series of the outer nuclear layer are 

 shriveled. In some eyes the ganglionic nuclei have for the greater part lost their 

 granular structure and show a homogeneous pasty condition, only a few cells 

 with granular nuclei being present (fig. lo/). The same is true in large part of 

 the inner nuclei of the inner nuclear layer. This condition of the ganglionic nuclei 

 is not entirely confined to the adult but is also found in the larva. 



Some of the modifications in the shapes of the outer nuclei in the adult are 

 shown in the figures. In figure ii b the upper part of the nucleus is very much elon- 

 gated. This form is of frequent occurrence. In figure ii c is shown the common 

 form where the nuclei are simple elliptical bodies, which gi\e no evidence what- 

 ever of any processes uniting them with the other elements of the retina. The Miil- 

 lerian fibers are profusely present and of very large size in both larva and adult. 



In both adult and young the optic nerve enters as a single strand and passes 

 entirely through the layers. A heavy mass of pigment is found following the optic 

 nerve to within a short distance of the brain. 



Average Measurements of the Eyes of Typhlotriton. 



Vertical diameter of eye 



From front of lens to back of eye. . . . 

 OuttT nuclear layer with the rods. . . . 



Outer reticular l.iyer 



Inner nuclear layer 



Inner reticular layer 



Ganglionic layer 



Pigment layer 



Optic nerve 



Lens 



35 

 mm. 



8lo 



600 



76 



I 

 76 

 16 

 68 



4 

 20 



342 



48 

 mm. 



800 



672 



42 



2 



72 



20 



56 

 16 



25 

 300 



Length of Specimen. 



62 

 mm. 



80 

 16 

 64 



960 



720 

 3" 



5° 

 24 

 32 



500 



07 

 mm. 



28 

 48 



8 

 24 



432 



103 

 mm. 



800 



720 

 28 



72 

 8 

 2C> 

 20 

 23 

 430 



106 

 mm. 



1 1 70 

 1134 



72 

 13 

 26 

 22 

 29 

 504 



CONCLUSIONS AS TO THE EYE OF TYPHLOTRITON SPEL/EUS. 



(i) The eye lies just beneath the skin. The skin is but little thinner over the 

 eye than elsewhere and shows no structural characters different from those of 

 neighboring regions. 



(2) The eye muscles have vanished. 



(3) The lens has vanished and its place has in part become filled by an ingrowth 

 of clioroidal tissue containing pigment. 



(4) The vitreal body is very small, if present at all. The vitreal cavity is a 

 funnel or trumpet-shaped space. 



(5) The pigmented layer of the retina is a pavement epithelium with indistinct 

 cell boundaries and with occasional pigmented processes extending into or through 

 the nuclear layers. 



