4o8 MONTGOMERY. [Vol. XIII. 



The nerve tubule is best studied in its proximal portion 

 (Figs. 3-14), since it has the greatest diameter at this part; 

 and further, owing to its somewhat tortuous course within the 

 fibrous core, it is only this portion which may be traced to the 

 ganglion cell (cf. Fig. 3). In a small number of cases I have 

 found spongioplasmic strands or fibres prolonged from the 

 cytoplasm of the cell for a short distance into the hyaloplasm 

 of the axis cylinder (Figs. 4, 5, 7, 11, 13, F). These strands of 

 spongioplasm are found only in the most proximal portion of 

 the axis cylinder, and are not continued distally. The obser- 

 vation of such fibres as these might have led Nansen ('87) to 

 the conclusion that the axis cylinder represents a bundle of 

 "primitive nerve tubes," and that these fibres correspond to 

 their supposed sheaths. For my part, however, I consider that 

 the nemertean axis-cylinder process represents a single nerve 

 tubule, and not a bundle of parallel "primitive nerve tubes." 

 And the spongioplasmic strands or fibres which I have described 

 in the axis cylinder, (i) being found in but a small proportion 

 of the large number of axis cylinders examined, (2) being even 

 then present only in their most proximal portions, and (3) vary- 

 ing both in number and size (Figs. 4, 5, 7, 11, 13), would lead 

 to the justifiable conclusion that they do not represent sectioned 

 sheaths of " primitive nerve tubes," but are merely spongio- 

 plasmic prolongations of the cytoplasm of the ganglion cell, 

 which penetrate but a short distance into the hyaloplasm of 

 the axis cylinder. 



On cross section, studied with high powers of the microscope, 

 the nerve tubule appears as a minute, circular, unstaining disc, 

 bounded by a fine, staining line (Fig. 39, Ax. CL, also Figs. 



36, 37)- 



As noted above, the nerve tubule has the greatest diameter 

 proximally, and distally gradually decreases in size (Figs. 

 3, 4). There are apparently no dichotomic divisions by which 

 two branches of equal diameter are produced. But the diminu- 

 tion in calibre is due, partly at least, to the giving off of collat- 

 erals from the nerve tubule, which always have a smaller diameter 

 than that of the nerve tubule at the point of division (Fig. 39, 

 Coll.). I have never been able to trace these collaterals for 



