SMITH: EYES OF PULMONATE GASTEROrODS. 2G1 



nearly parallel coarse. How much of a network they make here it is 

 very difficult to say, for they are much crowded. There are places 

 where there is an appearance of long meshes, but the knots certainly 

 give rise to few fibrils, and they might easily be interpreted as simply 

 the fusions of crossed fibrils. Figure 40 shows another illustration of a 

 neurite-process, but it is cut off before it narrows down into the neurite 

 proper ; the cell body, attached to the large end of the process, has not 

 been drawn. 



It is very difficult to make out how many fibrils pass down the neu- 

 rite, either in the accessory retina or in the chief eye. For the neuritcs 

 run in strands which it is difficult to separate. Moreover, if short bits 

 of single neurites are chosen, it is very difficult to be sure that they are 

 not radiculae. Cells with long stretches of neurite, such as are shown in 

 methylen-blue preparations, are very difficult to pick out in vom Rath 

 preparations. There is a lack of sharp differentiation of parts, which 

 makes the tracing of neurites difficult. Superimposing drawings of suc- 

 cessive sections has not yielded any satisfactory results in a study of this 

 matter. Before the use of methylen blue showed me how small the 

 neurites really are, an examination of vom Rath preparations made me 

 believe that I could speak much more confidently of fibrillae in the neu- 

 rites. I shall return to this aspect of the subject later. From prepara- 

 tions made by the vom Rath method, I have nothing to report as to the 

 neurofibrillae in the strands which within the eye capsule pass toward 

 the optic nerve, for the neurites usually appear like single, fine, dark or 

 black lines. They are very fine as compared with the axis-cylinder of 

 the vertebrate neurite. In the optic nerve vom Rath preparations give 

 some evidence of fibrillae, which I at first thought very good. A longi- 

 tudinal section of the nerve shows, now and then, more or less nearly 

 parallel, sinuous, black lines. They are probably neurites which have 

 been impregnated more heavily than their neighbors. Parallel with and 

 between them are close-packed, dark-gray lines of about the same tone as 

 the mantle-fibrils of the rod and of the same extreme degree of fineness. 

 I believe that these fine lines represent neurofibrillae. It is possible, 

 however, to interpret them as many, fine, closely-packed semi-trans- 

 parent neurites whose superimposed outlines give a fibrillar appearance? 

 for, as I have said, the vom Rath fluid, unlike the methylen blue, 

 impregnates all of the neurites. 



In the chief retina the fibrillae are visible only in the rods and in that 

 part of the sensory cell which is in the peripheral zone. In the latter 

 they are far more difficult to study than in the accessory retina, because 



