EYES OF CHOLOGASTER PAPILLIFERTJS. 113 



The optic nerve enters the retina as a single strand. It spreads out in all direc- 

 tions as soon as it has passed the pigmented part of the retina (fig. 35 c). Some of 

 the fibers pass behind the ganglionic cells just within the entrance of the optic 

 nerve, a condition of importance in the interpretation of the distribution of the optic 

 nerve in the blind members of the family. The diameter of the nervous opticus at the 

 entrance into the pigment layer is 32 p in the largest specimen. The nerve is not 

 spread out over the ganglionic layer, but is distributed in well-defined tracts between 

 the nuclei. There is no nerve fiber layer proper (figs. 35 c, d). These strands of 

 fiber not only entirely displace the ganglionic cells along their track, but also plow 

 into the granular layer. 



The pigment layer of the retina is very thick, as compared with the other layers, 

 a condition recalling that described by Ritter for Typhlogobius and usually to be 

 found in degenerate eyes. 



For a comparative statement of the thickness of the various layers of the retina, 

 see table on page 120. 



The pigmentary layer is half the total thickness of the retina in the smaller 

 specimen, while in the largest it is still thicker, measuring 104 of the 168 /a of the 

 retinal thickness. 



About one-eighth of the outer part of this layer usually appears as a solid mass 

 of pigment where the margins of the cells touch. Just within this is a region where 

 the cells are contracted, there being large, open, pigmentless spaces; at the inner- 

 most part there is again an accumulation of granular or rod -shaped pigment granules 

 which obscure almost everything else in the ordinary sections. (See fig. 35 c.) Speci- 

 mens preserved in chromic acid lose most or all of their pigment, which becomes 

 brownish or disappears. The nuclei of the pigment cells are very irregular in outline 

 (fig. 35/), appearing to have no more definite shape than those of white blood 

 corpuscles. Hollow processes extend from the cell body downward to near the 

 external limiting membrane (figs. 35 e and/). 



About the bodies of the cones the pigment is in thin strands, of which there are 

 8 to 12 to each twin cone; farther out it forms a complete ring about them. The 

 cones are twins, rarely triplets. The twins are nearly all arranged in such a man- 

 ner that the line, which may be termed the axis, connecting the centers of the com- 

 ponents of a twin are nearly parallel and form approximately part of an equatorial 

 circumference of the eye (fig. 36 b). There is, therefore, no resemblance to the 

 condition found in Coregonus and Zygonectes even if we omit for the present the 

 consideration of the rods (or single cones). The cones consist of an outer segment 

 (80 ix long in the largest specimen) with a tendency to become oblique near their 

 outer ends. In chromic preparations these readily split into disks. They stain 

 faintly but evenly. They are joined by a translucent interval to the body of the 

 cone, an ellipsoid body 5 /"• + 10 /u. taking on a deep stain (fig. 36 a). These rest 

 apparently on a membrane cylinder extending from their base to near the external 

 limiting membrane, a distance of 10 /*. Here they rest on a deeply staining cone- 

 shaped cell body which pierces the external limiting membrane and is extended 

 as a less deeply staining, nodulated process to the outer reticular layer, where it 

 spreads out into a cone-shaped base. 



The rods or single cones are very much fewer in number and not regularly 

 arranged. They are much fewer than the number of nuclei in the outer unclear 



