ITS ONTOGENY, MORPHOLOGY, PHYLOGENY, AND FUNCTION. 407 



IV. THE FINER ANATOMY AND INTERNAL STRUCTURE OF THE TORUS. 



t. Neuroglia. The torus longitudiualis has a strong radialframework of ependy- 

 imal fibres, of the same general type as those in the tectum from which the torus is 

 derived. These neuroglia elements are most numerous and best developed near 

 the median plane, which always retains to a considerable extent its primitive epen- 

 dymal structure. The cells are small, conical, or cylindrical in form, and located on 

 the convex boundary of the torus. Their internal ends are prolonged into long fibrils 

 of varying degrees of coarseness which extend dorsad along the radii of the torus 

 lobes towards the point of attachment to the tectum. Those arising from the sulcus 

 between the torus lobes are usually coarser. The ependymal fibres are most con- 

 spicuous at the anterior end where they radiate from the recessus above the posterior 

 commissure, dorsad and laterad, through the torus lobes. These ependymal fibres 

 were first seen by Sanders ('78), who, failing to recognize their true nature, described 

 them as radiating "bundles of fibrillae." Herrick ('91) failed to distinguish these 

 ependymal fibres from the nerve fibres issuing from the torus, and classed both sets 

 of fibres together as the "gelatinous tracts." They were finally correctly described 

 by Sala ('95) from his Golgi impregnations. 



2. Torus Cells. The cells of the torus are almost uniformly of small size. They 

 are generally arranged in parallel rows or series (Figs. 1, 2) between the radiating 

 ependymal fibres. But the definiteness of this arrangement varies greatly in different 

 species. This serial arrangement is most conspicuous where the ependymal fibres 

 are most numerous; in the lateral portions of the torus it is not apparent. 



In general the cells are spherical or ovoid in form, but the form is subject to varia- 

 tion. In the brook trout they are spindle-shaped and bipolar, the long axis extending 

 vertically. The nuclei are relatively large, making up the greater portion of the 

 cell-body. The cytoplasm is small in amount, stains lightly, and is not sharply out- 

 lined except at the poles of the cell where the neurites pass off. The cells are usually 

 bipolar, but may be unipolar or multipolar. In every case, however, three neurites 

 ultimately arise from the cell, either directly, or indirectly by the division of a chief 

 process. 



Sanders ('78) first described the cells of the torus in Mugil as follows: "The 

 cells which constitute the fornix are mostly of a spherical form, consisting almost 

 entirely of nuclei with only a narrow rim of protoplasm around them. . . . Occa- 

 sionally larger cells occur which present a triangular shape from the greater quantity 

 of protoplasm belonging to them." I have failed to find the second class of cells 



