No. 3-] STUDIES ON THE HETERONEMERTINI. 42 1 



distance in the fibrous core, and is always to be distinguished 

 from the really nervous elements by (i) its granular, refractive 

 appearance, (2) its staining power with eosin, (3) its irregular 

 branches. On cross section, after fixation in sublimate, such a 

 fibre appears as a refractive, staining granule or group of 

 smaller granules (depending upon its size). 



The nervous elements in the fibrous core, or dotted substance, 

 are the nerve tubules, and their course and grouping is to be 

 best studied on sections of material fixed with Hermann's fluid 

 (in Figs. 36-38 I have tried to reproduce the appearance of such 

 preparations, though the ground color should be more of a 

 bronze than green). The axis cylinder of the nerve tubule 

 never stains (except with methylene blue, and probably with 

 gold chloride: cf. Apathy, '91), is circular on cross section, and 

 its fine, staining envelope is the spongioplasmic sheath. After 

 fixation in Hermann's fluid for half an hour, when studied with 

 high magnifying powers (Fig. 39), the fibrous core is seen to 

 consist of a deeply stained, very finely granular matrix (of 

 neuroglia fibres), throughout which the unstaining axis cylinders 

 lie; with a lower power of magnification (Figs. 36-38) the 

 staining matrix appears homogeneous, deeply staining, and the 

 axis cylinders as fine, colorless lines, or dots on cross section. 

 Sublimate produces a more coarsely granular appearance of the 

 fibrous neuroglia matrix than does Hermann's fluid, probably 

 owing to the faculty of the former reagent to produce coagulation. 



In the fibrous core of the dorsal (Fig. 37) and ventral lobes 

 of the brain and in the dorso-ventral commissure the unstaining 

 axis cylinders are more or less regularly distributed, and are not 

 united into special bundles. In the dorsal commissure of the 

 brain (commissure of the dorsal lobes) and in the ventral com- 

 missures (Fig. 38), they all take a parallel course, but here also 

 are not grouped into bundles. 



There is in the lateral chord, however, a more specialized 

 arrangement of the nervous elements (Fig. 36). Here, namely, 

 a short distance behind the ventral lobe of the brain, about the 

 region of a frontal plane passing through the posterior end of 

 the cephalic sense organ, a large tube arises (Fig. ■^^6, N. T.) 

 which is situated a little lateral from the center of the fibrous 



