MUSKUM OF COMPARATIVE ZOOLOGY. 65 



tinguished next to the inner reticular layer. These arc slightly larger, 

 on the average, than are the more superficially situated nuclei (st. nl. ex.), 

 and they also stain somewhat more deeply. Usually each nucleus has 

 a centrally placed nucleolus, and a not very distinct nuclear membrane. 

 These nuclei undoubtedly represent the inner nuclear layer, though 

 whether the entire layer or only its spongioblasts, it is impossible 

 to say. 



Following this layer there is usually a single layer of nuclei {st. rtl. ex.) 

 that are considerably smaller and letis distinct than those of the layer 

 last described, Not only are the nuclei themselves here smaller and 

 less distinct than those of the layers on either side of them, but spaces 

 {st. rtl. ex.') are seen at intervals in which there are no nuclei at all. It 

 seems quite cei'tain that the outer reticular layer is represented by 

 these spaces. 



Finally, outside of this layer follows another of nuclei about one or 

 two deep (st. nl. ex.) that are again somewhat larger and more distinctly 

 stained than those in the layer last described. No difference between 

 them and those of the inner nuclear layer can be discovered, excepting, 

 as said above, that they are slightly smaller, and stain a little less 

 deeply. They are undoubtedly the nuclei of the rods, i. e. the outer 

 nuclear layer, though I have been unable to trace a connection between 

 them and the rods, and it is somewhat surprising that they are slightly 

 greater in diameter than the rods. 



The external limiting membrane (mb. lim. ex.) is usually quite 

 distinct. 



The rods are well — probably normally — developed, but I have 

 searched in vain for cones. In the retina shown in Figure 18, they are 

 quite variable both in length and diameter, being in a few instances 

 knobbed at th& outer ends {hac. da.). It is, however, quite possible 

 that both this appearance and the shortness of some of them are due 

 to artificial causes, but the variation in diameter could scarcely be so 

 explained. 



With a single exception, to be described more minutely hereafter, 

 the lens has been present in all the specimens studied in detail. 



It differs in no way in structure from the lens in normal fish eyes that 

 I have examined ; i. e. it appears entirely homogeneous and structure- 

 less after preservation in alcohol, Perenyi's fluid, or picro-nitric acid. 

 It is held in position very loosely, and consequently is easily displaced ; 

 it is frequently found, in prepared specimens of the eye, pushed entirely 

 out of its proper situation into the somewhat yielding connective tissue 



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