4IO MONTGOMERY. [Vol. XIII. 



axis cylinder, e.g., no alternating, concentric, or linear zones of 

 these substances. In short, the nerve tubule in the proximal 

 and distal portions of its extent consists, as in Lineus, of a thin 

 spongioplasmic sheath and an enclosed, homogeneous, unstain- 

 ing, hyaloplasmic axis cylinder; only in a small number of cases 

 are spongioplasmic fibres or strands, which vary in size and 

 number, continued from the framework of the cell for a short 

 distance into the hyaloplasm of the proximal portion of the axis 

 cylinder. 



The nerve tubules of cells I are so minute in diameter that 

 their structure can only be studied in their wider, proximal 

 portion (Figs. I, 17); as far as I could determine, they have the 

 same formation as those of III. In Fig. 37, representing a 

 cross section of the fibrous core of the dorsal brain lobe, the 

 unstained axis cylinders are probably of both cells I and III 

 (these two types of cells forming the ganglion-cell layer of the 

 dorsal lobe). The structure of the nerve tubules of cells II 

 (Figs. 2, 18) may be more easily studied than those of I, and 

 have exactly the same structure as the nerve tubules of III, 

 from which they differ only in size. 



It will be well to review briefly Burger's representations of 

 the structure of the nerve tubules of cells III before describing 

 those of IV. 



Burger's observations ('90b, '9lb, '95) differ quite essentially 

 from mine, though his direct observations less so than his gen- 

 eralizations. For the terms " spongioplasm " and "hyaloplasm " 

 used here, he adopts Flemming's expressions, "mitom" and 

 "paramitom." The following quotations will show his views 

 on the ganglion-cell processes ('95) : " Die Fortsatze der Gan- 

 glienzellen, welche den Stammfortsatzen von Retzius ['90] 

 entsprechen, zeigen mit Ausnahme derjenigen der Neurochord- 

 zellen perlschnurartige Verdickungen " (p. 332). . . . "Die 

 Nervenfaser der Central substanz d. i. der in ihr enthaltene 

 Abschnitt des Stammfortsatzes stellt namlich nicht einfach den 

 in den Seitenstamm der Nemertine verlangerten Fortsatz der 

 Ganglienzelle dar. Denn der Fortsatz einer Ganglienzelle 

 verjiingt sich in der Centralsubstanz oft bis in eine recht feine 

 Spitze und tritt mit dieser an die Nervenfaser heran, die haufig 



