406 THOS. H. MONTGOMERY jr., 
always stains intensely and is rich in chromatin (Chi. N); these nuclei 
are generally flattened and much twisted, so as to resemble much 
crumpled and wrinkled leaves, very different from the evenly outlined 
nuclei of the chromophobic nerve cells. The cell is multipolar, and 
stains intensely black with iron-haematoxylin. There is around the 
nucleus no noticeably thickened cytoplasmic enlargement, but from this 
region long branching processes pass into the fibrous core of the 
nerve cord and into the neural lamella. These processes are ex- 
ceedingly long, dense and smooth in contour, and are as clearly 
marked as lines of ink upon the preparations. The Figs. 22, 24, 25, 
Pl. 38; Fig. 26, Pl. 39 where these cells and their fibres have been 
carefully drawn with the camera lucida show these relations better 
than any description. Whether the branching fibres of one cell inter- 
lace with those of others or anastomose with them, could not be 
determined. These cells are fewer in number than the chromophobic 
cells, and their nuclei are for the most part ventrally situated; where 
transverse fibre commissures (Fig. 26 N.C.Com) occur the nuclei are 
also found upon the dorso-lateral margins of the cord. The greater 
part of the substance of the nerve cord is constituted by their rami- 
fying fibres. 
These cells would appear at first sight to be similar to the multi- 
polar neuroglia elements of certain other Invertebrates, such as the 
Nemerteans, that is, to he the supporting elements of the nervous 
system. But it will be shown that their processes pass through the 
neural lamella into the hypodermis and there stand in connection with 
sensory elements, so that they must be ascribed a sensory function in 
addition to one of mere support. 
The fibre tracts. On first examination the ventral cord ap- 
pears to be tripartite, composed of three rays converging to the point 
of the neural lamella. But a closer examination shows that each 
lateral ray is made up of several distinct fibre tracts, so that the 
following regions may be distinguished after a careful comparative 
study of the cord on transverse and longitudinal sections: 
1) The median tract (M.T Figs. 22, 24, 25, Pl. 38; Figs. 26, 
32, Pl. 39), the largest, extending from the ventral to the dorsal 
margin of the nerve cord, and bounded on either side by chromo- 
philic nerve fibres which course vertically. This tract is made up 
mainly of longitudinally directed chromophilic fibres, which are here 
more closely arranged than elsewhere in the cord. Only very rarely 
