34 SYDNEY EVANS JOHNSON 



In specimens 65 to 70 mm. long the entire sensory canal sys- 

 tem is completely mapped out. The canals are all closed and 

 have reached then* ultimate distribution, though in subsequent 

 stages they become somewhat greater in diameter and, on the 

 head, sink deeper in the integument, or even to a depth beneath 

 the integument. 



The differentiation of the sensory epithelium could not be 

 studied in Mustelus owing to the lack of sufficiently early stages. 



B. SQUALUS ACANTHI AS 



1. Development of the sensory canal system 



a. Early stages of development — 9 to 36 mm. In tracing the 

 development of the canal system of Squalus it will be convenient 

 to consider the matter under two heads: (a) early stages of 

 development, from the first definite appearance of sensory 

 cords on the surface to the time the lateral cords have grown 

 back to the tail, and (b) later stages of development, involving 

 involution of the sensory cords, the closure of the canals and 

 the differentiation of the sensory epithelium. For observa- 

 tions of the earlier stages I had embryos between 9 and 36 mm. 

 body length and for the later stages those from 40 to 72 mm. 

 The descriptions are confined chiefly to the development of the 

 lateral cord and canal, since their growth is typical of the cor- 

 responding structures of the head, simpler for examination and 

 not complicated by cephalic modifications. 



Material was not available for independent study of the first 

 phases of development of the canal system, but sketches of 

 Squalus embryos from 3.25 to 8 mm. length made under the 

 direction of Professor Locy by Mr. Otto Swezey seem to in- 

 dicate that the epithelium of the sensory cords has a common 

 origin with the epithelium of the ear. In tracing the primordia 

 of the lateral line organs two things are to be kept in mind — 

 the epithelium out of which the sensory cords are developed 

 and the primordia of the ganglia. One comes from the surface 

 ectoderm and the other from the neural crest. While the 

 ganglia arise independently, the sketches of Mr. Swezey appear 



