4o6 MONTGOMERY. [Vol. XIII. 



outer spongioplasmic sheath of the nerve tubule immediately 

 envelops the axis cylinder, which cylinder composes the greater 

 part of the tubule, and is a direct continuation into the latter 

 of the hyaloplasm (HI. PI.) of the cell. Thus the axis-cylinder 

 process is a 7ierve tubide and not a nerve fibril, since by the 

 term " fibril " is implied a very fine thread of more or less 

 dense (i.e., non-fluid) composition, which is also of equal density 

 in its whole diameter. In short, the typical structure of the 

 ganglion-cell process in Lineus is that of a nerve tubule, con- 

 sisting of a very fine-staining, fine-grained, spongioplasmic 

 sheath, which envelops the unstaining, hyaloplasmic axis 

 cylinder.^ The hyaloplasmic axis cylinder never stains with 

 any of the methods employed by me; an exception is found in 

 the ijitj'a vitam methylene blue staining; thus Apathy's ('91) 

 investigations show that the axis cylinder (his " Primitivfibrille ") 

 stains as well as its sheath; and since Burger's ('9ib) Fig. i8 « 

 shows that both hyalo- and spongioplasm stain within the gan- 

 glion cell, it is allowable to conclude that both the axis cylinder 

 and its sheath become stained with methylene blue. I might 

 also refer to my Fig. 41, from a preparation of Tetrastemma 

 fixed with sublimate and afterwards stained with aq. sol. of 

 methylene blue ; here the structure lettered N. T. ?, which prob- 

 ably represents a nerve tube (i.e., a bundle of nerve tubules), 

 has become deeply stained throughout with the methylene 

 blue. Further, the axis cylinder is always non-refractive 

 and homogeneous, and does not contain longitudinal " primitive 

 fibrils," nor other fibrous or granular structures; nor yet have 

 I found any evidences of a meshwork structure, as described 

 by Biitschli ('94) . The substance of the axis cylinder is always 

 homogeneous and structureless, and is identical with the hyalo- 

 plasm of the cell. This hyaloplasm must be a fluid, or at least 

 a viscid, substance in life ; but it certainly cannot be dense, 

 since in that case we would expect to find it refractive, and to 



1 The very fine membrane around the axis cylinder in vertebrate nerve fibres, 

 as figured by Koelliker ('89), might correspond to such a spongioplasmic sheath 

 as that described bv me in nemerteans. The "primitive nerve fibrils," such as 

 have been described for a large number of forms, lie within the substance of the 

 axis cylinder. 



