EYE OF STYGICOLA. ' 225 



I am not sure whether the conditions seen in older eyes are to be taken as the 

 result of retrogressive changes, or of an abortive differentiation of an outer reticular 

 layer with a separation of the nuclear into an outer and inner nuclear layer. 



The definiteness of the reticular and outer nuclear layers is no longer found in 

 older fishes. Instead, there is found an irregular series of cells bounding the epi- 

 thelial face of the retina (plate 26, fig. a). The other nuclei of the outer layer, seen 

 in younger fishes, are scattered irregularly through the retina, leaving, however, a 

 distinct, inner reticular layer. The outer reticular layer (4) so formed is much 

 thicker than in normal retinas, and it is otherwise so irregular that it can scarcely 

 be considered the homologue of the outer reticular layer of normal retinas. 



In the oldest eye examined even this degree of regularity is gone (plate 26, fig. f). 

 There is an undoubted reduction in the number of nuclei, remains of which are seen 

 as dark granules among the nuclei. The character of the nuclei in the older indi- 

 viduals differs ; some still show granules, while others stain uniformly. 



The optic nerve is very well developed in the young 22 mm. long, and can be as 

 readily followed to the brain as in normal eyes. It becomes proportionately more 

 slender with age. It is observable even in the oldest eyes, but in them it has been 

 impossible to trace the optic nerves outside the bulb. 



