SMITH: EYES OF PULMONATE GASTEKOPODS. 273 



prism-like middle part of the coll. At certain points in the focusing the 

 sharp edges of the prism may come to view in such a way as to be very 

 deceptive. There is great danger of mistaking these sharp edges for 

 parallel neurofibrils. One who has caught a glimpse of the parallel 

 fibrils of the rod as they enter the pigment zone (as in Plate 4, Fig. 48) 

 might very naturally conclude that the fibrils, although invisible, con- 

 tinue through the cell in the same way as through the rod. Even when 

 the fibrils are brought into evidence, as in the vom Rath method, which 

 gives a coufusion of fibrils, the same reasoning would lead to a like 

 conclusion. Influenced by these latter considerations, my preliminary 

 sketches of the sensory cells of the accessory retina showed parallel 

 fibrils passing directly through the cells. It was only after prolonged 

 study of the cells under high magnification that I was forced to con- 

 clude that the vom Rath preparations showed a network in the vicinity 

 of the nucleus. This conclusion was subsequently reinforced by the use 

 of the methods of Bethe and Prentiss, which differentiate only a part of 

 the fibrils, thus making the study more easy. The chances of error are 

 so great in the study of fibrillae that special methods should be used 

 wherever possible. 



"Whatever may be the facts in other gasteropods, all of the evidence 

 from Limax seen in the microscope is in favor of a network of fibrils in 

 the body of the sensory cell. Other considerations show how untenable 

 is the view that the separate fibrils of the core of the rod find their way 

 directly into the neurite. I have shown that the neurites of the optic 

 nerve in Limax are very fine. Even if they contain more than a single 

 fibril, they can scarcely contain as many fibrils as there are in the core 

 of the rod. Large cells with many fibrils, as in the accessory retina, 

 are not represented in the optic nerve by larger neurites than the. smaller 

 cells with fewer fibrils, such as are found in many of the cells of the 

 chief retina ; but even if they were, it can be shown that the total area 

 of the cross section of all the fibrils in the core of a rod much exceeds 

 the area of a cross section of a neurite in the optic nerve. This last 

 point would hold even if the core fibrils were as fine as the mantle- 

 fibrils. I have already explained why the network cannot represent 

 cross sections of alveoli ; and it is most unlikely that fusions of sinuous, 

 though distinct, fibrils would persistently and consistently produce in 

 three different methods of treatment the appearance of a network, even 

 in combination with an alveolar system. It is probable, therefore, that 

 there takes place within the cell a large alteration (increasing distally, 

 decreasing proximally) in the number of fibrils. 

 vol. xlviii. — No. 3 18 



