40 CHARLES F. W. MCCLURE, 



The fibrils in this form are plainly seen and can be traced for 

 quite a distance into the cell body, even in thick sections (5 fi). 



In addition to the fineness of these fibrils in Helix and Arion, 

 another circumstance may account for their indistinct definition in 

 relatively thick sections of the axis-cylinder process of the cells in 

 these species. This is the circumstance that the fibrils lie exceedingly 

 close to each other, on account of the small amount of ground-sub- 

 stance present in this region. 



DoGiEL (7) suggests the same cause in explanation of a similar 

 difficulty experienced by him, in observing fibrils in the axis-cylinder 

 processes of certain Vertebrate nerve cells. He says on p. 412: 

 "Der Grund, warum im Axencylinderfortsatz die fibrilläre Structur 

 schwieriger zu bemerken ist als im Protoplasmafortsatz, ist dem 

 Umstand zuzuschreiben, dass in ihm sehr wenig Grundsubstanz vor- 

 handen ist, und deshalb die zu seinem Bestand gehörigen Fäden ein- 

 ander ausserordentlich nahe anliegen." 



This general lack of ground-substance and close arrangement of 

 the fibrils, seem to hold good for the axis-cylinder processes of all 

 Invertebrate nerve cells examined in the course of this investigation. 

 By substituting the term "Körper der Zellen" for "Protoplasmafort- 

 sätze", in the preceding quotation, the latter serves, in part, as ex- 

 plaining the condition found by me in the nerve cells of Avion and 

 Helix. However, in addition to this lack of ground-substance and 

 close arrangement of the fibrils in the processes, the smallness of the 

 fibrils is unquestionably an important factor in preventing their clear 

 definition in any but thin sections. Otherwise, it is difficult to account 

 for the distinct manner in which they are brought out in the axis- 

 cylinder processes of other Invertebrates. 



We pass now to a more detailed account of these fibrillar 

 structures. 



Fibrils of the Axis-Cylinder Process. 



So far as my experience is concerned, these fibrils are most 

 clearly shown in material which has been fixed in Flemming's solution 

 and stained by the iron-alum-haematoxylin method. In thin sections 

 of material prepared in this manner (2 to 3 ^i), these fibrils, which 

 are delicate threadlike structures and much finer than the coarse 

 neuroglia fibrils surrounding the cell, are seen running in a direction 

 parallel to the long axis of the cell process and frequently in wavy 

 bundles (Figs. 11, 12 and 13). 



