66 SYDNEY EVANS JOHNSON 



Structure. Differences of considerable extent are found in 

 the form of the canals and in the histological structure of the 

 sensory epithelium. 



The lateral canals lie in the deeper layers of the dermis and 

 their tubules pass ventralwards for a considerable distance 

 (0.1 to 0.5 mm. or more, depending upon their position) before 

 opening to the surface (fig. 80). The walls of the tubules con- 

 sist of an outer columnar and an inner flattened layer of cells. 

 These layers are continuous centrally ^\dth similar layers of the 

 canal walls and externally with the columnar and stratified 

 layers of the epidermis. 



In transverse section the sensory canals are usually round or 

 nearly so (fig. 82), and a small bundle of nerve fibers runs along 

 the base of the sensory column (fig. 81). The sensory column 

 lies in the dor so-medial wall of the canal (figs. 79, 80). 



The structure of the sensory column is indicated in figures 

 81, 82, 83. The hair-cells are arranged in small groups (about 

 3 to 5) around which the supporting elements tend to group 

 themselves in small concentric clusters. Usually only one 

 hair-cell can be seen in transverse sections. The cells are rela- 

 tively large and each appears to bear a single hair-like process 

 (figs. 82, 83). The supporting elements consist of columnar 

 cells, basal cells, and spindle-shaped cells. These are similar 

 to the corresponding elements found in the sensory column of 

 Mustelus. 



Efi"orts to secure silver impregnation of the peripheral termina- 

 tions of the lateral nerve of Squalus acanthias were not satis- 

 factory. Fresh Squalus material has not been available and 

 the fixation of the preserved specimens was not suitable for 

 silver impregnation methods. By other methods (non haema- 

 toxyhn and Weigert-Pal) , however, the lateral nerve has been 

 traced from its central ganglion to the basilar membrane of the 

 sensory column and it appears safe to assume that the termina- 

 tions are essentially similar to those of the lateral nerve in 

 Mustelus. 



The question of the mode of growth of the lateral nerve should 

 be considered. Regarding the growth of the lateral nerve, 



