smith: eyes of pulmonate gasteropods. 265 



35 (Plate 3). One neurite (the left in the figure) seems to he flattened 

 so that its diameter is increased at one region, and throughout this 

 region the neurite seems to be fibrillar. In another example (the cen- 

 tral cme in the figure) the neurite is cut and seems to be frayed out at 

 the end into several fibrils. In another place (at the right in the figure) 

 there was a larger number of fine, blue fibrils, which had much smaller 

 diameters than the neurites. It is my opinion that each neurite in the 

 optic nerve contains a few fine fibrils. 



I have given no examples of the fibrils of the rod stained by methylen 

 blue, for in that part of the sensory cell this stain is very unsatisfactory. 

 Sometimes the fibrils of the core were well differentiated, but in those 

 cases the whole rod was much swollen. Sometimes the mantle-fibrils 

 were sufficiently well impregnated to show that they are nervous sub- 

 stance. Sometimes by a curious selection the end bodies alone are 

 stained. 



General Discussions and Conclusions. 



"Without entering into controversy as to whether or not the sensory 

 cells of the retina, in the course of phylogenesis, have been derived from 

 the surface epithelium, it is evident from what has been shown in the pres- 

 ent research that the retina is an epithelium. Babuchin ('65) was the 

 first to show the single-layer condition of the retina, in regard to which 

 former investigators (Krohn, '37 ; Leydig, '57, p. 253; Keferstein, '62-66, 

 p. 970, Taf. 83, '64) had left only confusion. Leydig ('65) likewise came to 

 the belief that the various zones of the retina represented only one layer 

 of cells. But it was Hensen ('65) who saw more clearly than any of his 

 contemporaries that the retina is an epithelium one cell thick. At this 

 period, evidence of the epithelial nature of the retina was further in- 

 creased by many researches in the development of the eyes of molluscs, 

 which cannot be properly reviewed here. There are also several con- 

 tributions on the adult retina which substantiate the idea that the retina 

 is an epithelial layei". Bergh ('66), whose results I know only through 

 reviews of them by Hilger and others, showed that the cup-like eyes of 

 Fisurella represent simple, incomplete infoldings of the epidermis of the 

 surface of the body. Braun ('79, seen by me only in reviews) and 

 Fraisse ('81) showed that the degree of development of the eye of cer- 

 tain gasteropods (Patella, Haliotis, Fisurella) is an index of the phylo- 



