No. 3-] STUDIES ON THE HETERONEMERTINI. 407 



find it showing evidences of a finer structure. It is strange 

 that Apathy ('91), whose description of the axis cylinder (his 

 " leitende Primitivfibrille ") of the Hirudinea closely corresponds 

 to mine of the Nemerthii, should acknowledge that the axis 

 cylinder is homogeneous, and yet conclude that it is solid and 

 non-fluid in consistency. He reached this result as to its den- 

 sity simply from observations with polarized light; for the axis 

 cylinder would be capable of isolation, provided that its sheath 

 remained intact, even if it were fluid in consistency. Not, I 

 think, that the best reason for the correctness of the assumption, 

 that the substance of the axis cylinder has a fluid or viscid, and 

 not a dense, consistency, is to be given by the fact that this 

 substance is in every way identical with the hyaloplasm of the 

 cell; and all observations tend to prove that the hyaloplasm of 

 the cell cannot be of solid (dense) consistency, but is fluid, or 

 at least viscid. 



The structure of the nemertean nerve tubule, as just 

 described, may be seen after preparation with corrosive subli- 

 mate, but is rendered much more distinct after fixation in 

 Flemming's or Hermann's fluids. The fluid of Hermann 

 especially renders the course and structure of the nerve 

 tubules very plain, and for this purpose cannot be too highly 

 praised. Thus, after fixation of half an hour in this fluid, the 

 spongioplasmic sheaths of the tubules and all the neuroglia 

 fibres (or " fibrils," as one will) of the fibrous core of the brain 

 and lateral chords become stained a deep bronze-brown, while 

 the hyaloplasmic axis cylinders remain unstained, and are easily 

 traceable as fine, colorless lines. And this result is reached 

 after sectioning in paraffine and mounting in Canada balsam, 

 — a course of procedure much deprecated by Apathy ('91). 

 Hermann's fluid, accordingly, produces just the very opposite 

 effect of the impregnation method of Golgi: by the former 

 only the axis cylinders become stained, while by the silver 

 method it is especially these parts which become blackened. 

 But Hermann's fluid is always precise in its effect, while, as 

 Friedlander ('89) has shown, the Golgi method is far from reli- 

 able, since frequently neuroglia, as well as the strictly nervous 

 elements, become stained by it. 



