LATERAL CANAL SYSTEM OF SELACHIANS 71 



between the hair-cells. Owing to the lack of fresh material the 

 peripheral terminations of the lateral nerve in Squalus have not 

 been demonstrated, although the nerve fibers have been readily 

 followed as far as the basilar membrane of the sensory column, 

 and are similar in arrangement to those of Mustelus. 



The sensory canals begin their development as Imear ex- 

 tensions from certain thickened patches of ectoderm. Two of 

 these thickenings, the suprabranchial and the preauditory are 

 well defined. The points of origin of the mandibular surface 

 organs, and the surface organs which appear in front of the 

 yolk-stalk are not so evident. 



By linear extensions the suprabranchial thickening gives rise 

 to the lateral sensory cord, the dorsal series at pit organs, the 

 supratemporal commissure, and two isolated pit organs in front 

 of the pores of the endolymphatic ducts. 



Similarly the preauditory thickening gives rise to the supraor- 

 bital, infraorbital and hyomandibular sensory cords. 



As the sensory cords elongate in their respective directions 

 the ectoderm which lies in their path is pushed up to form little 

 pockets or even long tunnels over the growing ends of the cords. 

 These tunnels are not the canals. 



Early in their development cellular differentiation begins in 

 the sensory cords and by the time the thickenings have practi- 

 cally reached their full linear extension they begin to sink into 

 a groove at their proximal ends. As the infolding continues 

 the sensory cords are gradually enclosed wdthin epithehal canals 

 which, however, retain communication with the exterior by 

 means of numerous delicate tubules. 



By the time the suprabranchial thickening has become fairly 

 prominent and considerably before the lateral sensory cord has 

 definitely appeared, the suprabranchial ectoderm is separated 

 from the wall of the medulla by a mass of cells, the neuroblasts 

 of which give rise to the lateralis and vagus gangUa. No 

 evidence has been seen to indicate .that any of the cells in the 

 ganghonic mass are derived from the external ectoderm. This 

 point, however, has not been the subject of special examination. 

 While the external ectoderm is still in contact with the ganghonic 



