LATERAL CANAL SYSTEM OF SELACHIANS 25 



are not the same size throughout the length of the sensory col- 

 umn, but are considerably smaller in the posterior region (figs. 

 9, 11). They extend through about two-thirds the depth of 

 the sensory column, and their distal ends abut against a limit- 

 ing membrane which presents a great deal of variation in thick- 

 ness in different preparations. 



Basal ceUs. The boundaries of the basal cells are usually 

 difficult to trace, except their proximal margins, which rest on 

 a well developed basilar membrane. The cells usually appear 

 somewhat triangular in shape and they extend distally between 

 the proximal ends of the hair-cells. Their nuclei are round, 

 centrally located, and frequently appear almost as large as the 

 nuclei of the hair-cells (figs. 9, 11). 



The spindle-shaped cells. These cells occur between and 

 around the hair-cells (figs. 8, 9 to 12). They are very slender 

 and greatly elongated, extending from the basilar membrane to 

 the distal limiting membrane of the sensory colunm. The 

 nuclei are small, much elongated, and are located near the proxi- 

 mal ends of the cells except in cells which lie innnediatly between 

 two adjacent clusters of hair-cells; in these the nuclei are near 

 the distal ends of the cells (fig. 5). 



Columnar cells. The columnar cells are the most numerous 

 of all the cells in the sensory column. They extend from the 

 basilar membrane to the distal limiting membrane and vary in 

 length with the variation in thickness of the sensory column 

 (figs. 9, 10). On each side of the sensory column proper the 

 columnar epithelium becomes thinner and merges gradually into 

 the thin, inner epithelial layer of the sensory canal (figs. 7, 8). 



Fig. 8 Transverse section of lateral sensory canal, showing the histological 

 structure of the sensory column, peripheral nerve terminations, and the longi- 

 tudinal fiber zone (of nerve distribution). The figure was drawn with the aid 

 of a camera lucida from a pyridine silver preparation counterstained with Dela- 

 field's haematoxylin. Mustelus canis garter. X 370. 



Figs. 9, 10, 11 Transverse sections of the lateral sensory column of a small 

 adult Mustelus canis, showing that, posteriorly, the sensory epithelium is con- 

 siderably less extensive than anteriorly. Figure 11 was taken anterior to the 

 first dorsal fin; figure 10, between the first and second dorsal fins; and figure 9, 

 posterior to the second dorsal fin. X 370. 



