THE EYES OF AMBLYOPSIS. 143 



found in this layer, and I have not been able to identify Mlillerian nuclei as such 

 either in this or the nuclear layers. The ganglionic layer forms a compact mass 

 of nuclei, somewhat funnel-shaped, with the narrow end toward the exit of the nerve 

 (9 in figs. 50-54 e). I have found from 60 to 125 nuclei in this mass. At the 

 wide end of the funnel this mass of cells is directly continuous with the cells of the 

 nuclear layers. The cells in this intermediate layer are of the large type, and as 

 they give off fibers to the optic nerve, they may be classed as ganglionic or possibly 

 as cells belonging to the spongioblasts. 



Optic Nerve and Lens. — The optic nerve is always evident in the eye itself 

 except in very old individuals. It passes as a compact thread through the pigmented 

 layer into the ganglionic layer. Here it breaks up into smaller bundles, the fibers 

 of which pass in part to the cells within the ganglionic core, while the greater part 

 pass to the large cells at the outer rim where the ganglionic cells pass over into the 

 cells of the granular layers. The fact that these large cells give off the greater part 

 of the optic fibers suggests whether or not these cells are really the ganglionic cells, 

 while the cells forming the core are such cells as are seen at the entrance of the optic 

 nerve in Chologaster (z in fig. 35 c) and there form a plug around which the optic 

 fibers pass directly to the ganglionic cells. The bundles of fibers passing to the 

 anterior cells never pass through the mass of core cells but at one side of this mass. 

 In the right eye of an individual 25 mm. long they pass out in front of the mass ; 

 in the left eye of the same individual, behind them. 



Outside the eye itself the matter of following the optic nerve becomes a much 

 varying task. In very young, and up to 25 mm., there is no difficulty in tracing 

 the optic nerve to the brain. In newly freed individuals (about two months old) 

 the optic nerve passes nearly obliquely down and in, while in an individual 25 

 mm. long it passes horizontally back and in toward the foramen for the optic 

 nerve. In the latter individual the nerve leaves the eye, not as might be expected 

 at the posterior inner face, but at the anterior inner, making a sharp turn as it 

 leaves the eye. Its compact nature is entirely lost after leaving the eye, forming 

 a loose bundle several times as thick as the optic nerve within the eye. It is here 

 surrounded by a very thin film of pigment, which in its turn is surrounded by 

 layers of fibrous tissue. 



In individuals much more than 25 mm. long it is usually no longer possible to 

 follow the nerve to the brain. It can be followed some distance, but usually dis- 

 appears before reaching the optic foramen. In but one instance did I succeed in 

 following it into the brain cavity in an adult specimen. The structures surround- 

 ing the optic nerve are as variable as those surrounding the eye. In one case it is 

 surrounded by various layers of pigment, while in others scarcely any pigment is 

 found with it. 



The most highly differentiated lens * was found in an individual 130 mm. long. 

 i. e., a very old one. The lens in this case consists of a few nuclei about which 

 there are concentric layers of a homogeneous tissue (fig. 54/). In other individuals 

 structures approaching this condition were found (fig. 55 a), in one a large cell, 

 in another a cell with concentrically arranged lamellae. The lens, in an individ- 

 ual 25 mm. long, could not be found at all, and in another 35 mm. long could 



1 It is certain that this is not the lens. The name " secondary lens " may be applied to it. Similar structures 

 are found occasionally in Rhineura and Lucifuga. 



