Eyes of MoUuscs and Arthropods. 553 



completion , preparatious can bc made of the isolateci ommatidia in all 

 stages of disiutegratiou. The maceratiou must be complete, in order to 

 isolate the retiuophorae from their investing- pigment cells. When this 

 is suecessfuU}- donc. these cells, on account of their characteristic shape 

 and general appearance, can at once be distiuguished from all others. 

 They vary considerably in size, probably due to the fact that they come 

 from eyes in different stages of development , or from different parts of 

 the same eye. This Variation, however, does not afifect their character- 

 istic form, since they are in all cases wedge-shaped, with a blunt outer 

 end, and a pointed, inner extremity prolonged into a long, varicose 

 nerve fibre (figs. 52 and 59) . The trausformation of the inner extremity 

 of these cells into nerve fibre is so graduai , that it is impossible to say 

 v^here the one ends or the other begins. This is characteristic of the 

 retinophorae in all Mollusca, and is in marked contrast with the nerve 

 endings on the surrounding pigmented cover cells, er, indeed, on the 

 indifferent cells of the epithelium . If the maceration has been carried 

 so far as to separate the rods from the retinophorae , it will be seen 

 that the broad end is abruptly rounded, and that, at the very outer limits 

 of the cell, but always on one side, a neariy spherical nucleus is situated. 

 It is filled with many fine and deeply stained granules, while , in the 

 centre, is a small but distinct nucleolus. The nucleus is placed so close 

 to the cell wall, that it is often difficult to distinguish their respective 

 boundaries. On the side of the cell opposi te to that on which is situated 

 the nucleus. is an irregulär, rouudish and refractive body, absorbing 

 little coloring matter, but sufficieut to distinguish it as the abortive 

 nucleus of the second of the two cells composing the retinophora. It is 

 not always easy . and in many cases I have found it impossible, to sat- 

 isfy myself of the presence of this body, but the shape of the cell, and the 

 lateral position of the other nucleus, indicated that the retinophorae 

 were, in all cases, formed by the fusion of two cells , although it was 

 not always possible to distinguish both nuclei. The broad, outer end 

 of the retinophorae, — in the broadest part about .005 mm wide, — 

 is filled with a clear, finely granular protoplasm, a narrow area of struc- 

 tureless and refractive fluid surrounding the larger nucleus. The re- 

 maining portion of the cell is occupied by closely packed , transparent 

 and refractive globules, divided into two groups, an outer one composed 

 of larger globules, and an inner one of smaller ones. They are easily 

 destroyed by too much maceration, but, since a prolonged treatment is 

 necessary to separate these cells, it is difficult to observe this structure, 

 although I bave seen it often enough, and with sufficieut clearness, to 



