THE EYES OF AMBLYOPSIS. 



141 



pigment is either entirely absent or very sparingly developed. As mentioned 

 above, the pigmentation of the eye seems to vary inversely with the pigmentation 

 of the surrounding structures. 



The pigment is in all cases granular and differs in this respect from the pris- 

 matic pigment of the eye of Chologaster. 



Iris. — The pigment cells decrease in height toward the irideal part of the eye, 

 where they are replaced by a layer of pigmentless cells forming a thin membrane 

 (fig. 50). The nuclei of these cells stain darker than the bodies of the cells, 

 which is the reverse of the conditions seen in the pigmented cells. In individuals 

 up to 35 mm. long similar cells ex- 

 tend along the line of the vanishing 

 choroid fissure (figs. 54, a and /). 



The pigmentless membrane is ap- 

 parently the relic of the outer pig- 

 mented layer of the iris. If so it has 

 undergone greater changes than the 

 rest of the pigmented layer, for it is 

 well pigmented in all the species of 

 Chologaster. 



The inner layer of the iris is fre- 

 quently entirely separated from the 

 outer layer and not infrequently is 

 entirely obliterated. (A few rounded 

 pigment masses are always found 

 within the eye at this point.) In 

 other individuals a minute opening 

 is still present and the outer layer of 

 the iris is continuous with the inner, 

 which contains some of the elongate 



nucleated Cells found in the region Of FlO-,53- Horizontal Section through Right Eye of Specimen, 25 mm. 



long from above. A Large Branch of Optic Nerve is seen to pass 

 in front of Cone of Ganglionic Cells. This is not Constant, and 

 in Left Eye of Same Individual the Largest Strand passes 

 behind Number of Ganglionic Cells lying in front of Inner 

 Reticular Layer and the Central Ganglionic Mass. 



Typhlichthys subterraneus . These 



nuclei are variously grouped in different eyes, as is represented by the figures 50, 

 51 b, 54 b, d, e. The exact significance of the various structures about this region 

 in the eye can not always be determined owing to their presence or absence 

 in different individuals and their great variability when they are present. In this 

 region are sometimes a few cells with elongate nuclei that can not be identified with 

 any of the structures considered. These may represent all that is left of the hya- 

 loid. Blood-vessels are usually not found in the eye of the adult. 



Between this pigmentless membrane and the rest of the retinal structures, i.e. 

 within the pigment epithelium, there is in the majority of the adult eyes an irregular 

 mass of pigmented cells. I am entirely at a loss to account for this mass unless 

 with the shrinking of the eye as the result of the loss of the vitreous body and lens 

 and the consequent closing of the pupil, the margin of the iris is rolled inward 

 and some of the pigmented cells of the outer layer of the iris come to lie within the 

 eye after the closing of the pupil. The iris is seen to be rolled in the way imagined 



the ora serrata in Chologaster. These 

 are much more regularly present in 



