No. 3.] STUDIES ON THE HETERONEMERTINI. 395 



Fibrillen, die sich im Umkreis des Kernes, unter Bildung 

 zahlreicher Queranastomosen in ein Netzwerk auflosen. Die 

 Varikositaten sind eine besondere Eigenschaft der Plasmafibril- 

 len, da sie denjenigen des Achsencylinders fehlen oder doch 

 nicht durch die Farbung nachzuweisen sind. . . . Die Kern- 

 membran besitzt knotchenartige Verdickungen von gleicher 

 Beschaffenheit, wie diejenigen der Plasmafibrillen. Diese 

 Knotchen bilden die Vereinigungspunkte der sowohl vom 

 Plasma, als auch vom Kerngeriist ausgehenden Endfaserchen " 

 (P- 538)- In justice to that excellent observer Leydig must 

 be stated that he was the first to figure such varicose, concen- 

 tric fibrils ('85). What Pflucke describes as varicosities of 

 supposed concentric cytoplasmic fibrils (which he states com- 

 pose the " primitive fibrils " of the axis cylinder) correspond to 

 the chromophilic granules of other authors. A certain simi- 

 larity is to be noticed between his conclusions and those of 

 Nansen ('87). Thus Nansen, although he had not seen the 

 chromophilic granules, described the primitive tubes of the axis 

 cylinder as encircling the nucleus concentrically; the concentric 

 fibrils of Pflucke might correspond to the sheaths of such 

 primitive tubes, or vice versa. Pfliicke's paper reopens up a 

 field which certainly deserves reinvestigation. 



In the ganglion cells of Unio, H. Schultze ('79) found 

 "grossere, doppeltcontourirte, myelinahnliche Tropfen," which 

 became browned by osmic acid; if these really contain myelin, 

 which is doubtful, they would correspond neither to the 

 chromophilic granules nor to the chromophilic corpuscles. 



b. CerebraUihis. — The cells III in this genus differ slightly 

 from those of Linens and are considerably larger (Figs. 19-21, 

 29, ///) ; they resemble closely those described by Burger ('90 b) 

 for C. marginatiis. They are usually flask-shaped, swollen and 

 rounded proximally, distally tapering to the axis-cylinder pole ; 

 sometimes even retort-shaped, ovoid, or nearly spherical ; in 

 the lateral chords they may be even angular in outline. 



The nucleus (Fig. 22 a-e) is usually proximal or central in 

 position, seldom distal (this being the frequent position in 

 Lineus). It is usually nearly spherical or spherico-oval, some- 

 times kidney-shaped ; but I have never seen one of a horse- 



