^16 William Patten 



are formed by the fusion of two cells, one nucleus of which re- 

 tains its originai characteristics, aud may be readily seeu situated at the 

 base of the celi. The other micleus may. iu mauy cases, become so 

 degenerate as to entirely eseape uotice; but stili, in carefully stained 

 specimens. it at times appears as clearly defìued as in ordinary cells, 

 although it is usually seen as a homogeneous. faintly stained, ovai body 

 (figs. 66 and 67 n. rf.'^) at the outer end of the celi, in the centre of a 

 gentle swelling. Between the two nuclei, the cells are often constricted 

 to a narrow neck, or eveu to a slender fibre; the outer part terminates 

 in a narrowed portion filled with fine granules, which stain in haema- 

 toxylin, and give to this part a special prominence. 



When seen in cross sections , this granular part projects a short 

 distance beyoud the ends of the pigment cells (fig. 58 x). In one in- 

 stance, I found a very beautiful celi, the outer end of which 

 terminated in two separate pieces (fig. 66) furnishing a most 

 conclusive proofthat the retinophora. with its two nuclei, 

 was f r m e d b y the f u s i o n o f t w o cells. The celi contents consist 

 of a clear, protoplasmic mass in which are imbedded many granules of 

 various sizes. some of which are so refractive as to appear like pigment 

 granules. The distinction between the pigmented cells and the colorless 

 ones is not as sharp as in Arca^ for I found severa! that I was in doubt 

 whether to cali pigment cells or retinophorae. The inner ends of the 

 latter terminate in a relati vely large, varicose nerve fibre, into which 

 they are so gradually trausformed that it is difficult to teli where the 

 cells end and the fibres begin. One may follow the fibre in its course 

 through the centre of the celi, passing to one side of the large basai nu- 

 cleus, until it reaches the apex, where it issues agaiu aud passes out- 

 wards between the double rods of the retinophorae. Other nerve fibres 

 follow the wall of the celi to its outer extremity, where they appear to 

 terminate. The fibres often hang freely from the cells, so that one sees 

 they are provided with numerous extraordinarily fine fibrils (fig. 66) 

 exactly similar to those found in Arca. But the most conclusive proof 

 that nerve endings occur bere similar to those in Arca . was found by 

 treating the rods in such a manner as to d i s s o 1 v e t h e e u t i e u 1 a r s u b- 

 stance of the rods. leaving the mass of nerve fibres per- 

 fectly free aud iutact. To do this, the eyes were hardeued in 1% 

 bichromate of potash, for 24 hours, and then removed to dilute glyceriue, 

 1 part to 4 parts of distilled water ; a further treatment with strong gly- 

 cerine followed, after which they were washed in distilled water. Small 

 pieces may then be placed in acetate of potash or water, separated with 



