62 SYDNEY EVANS JOHNSON 



ner (fig. 73). Figure.s 74 and 75 show transverse sections of 

 the canal somewhat farther back. In figure 74 the trunk canal 

 is almost closed and in figure 75 it is still a wide open groove. 

 Figure 75 shows also an accessory organ which is now consider- 

 ably removed from the canal and is covered by a superficial 

 layer of ectoderm. 



Seventy-two millimeter stage. In this embryo (fig. 76) the 

 canals appeared to be completely closed and could be traced 

 only by the openings of the tubules. Anterior to the first dorsal 

 fin the lateral canal lies considerably beneath the epidermis, 

 its lumen is greatly enlarged, and its epithelial walls relatively 

 thinner than in previous stages (fig. 77). Posteriorly the canal 

 has not yet separated from the epidermis (fig. 78), the lumen is 

 smaller, and here, as in other sections (e.g., fig. 79) it is seen that 

 the colunmar layer of the canal wall is continuous \\dth the 

 columnar layer of the epidermis. 



In surface view the various fines of true canal organs are rep- 

 resented by continuous lines, the pores of the tubules being too 

 small for representation on the scale of the drawing. The ac- 

 cessoiy line organs, those of the dorsal line series and those in 

 front of the supratemporal commissm-e, are represented some- 

 what larger than their ratio to the outline of the embryo. 



The 72 mm. specimen was the most advanced of the embryos 

 at my disposal and therefore it completes my study of the 

 method of formation of the sensory canals. The developmental 

 processes are complete in all essential features, however, as may 

 be seen by a comparison with the conditions found in the adult. 



2. Structure of the sensory canals of the adult 



Distribution. Particular attention has not been given in my 

 observations to the distribution of the various sensory canals in 

 Squalus, although on rather superficial examination enough 

 differences have been noted to enable one to distinguish the 

 two genera by this character alone (Garman '88). In Squalus 

 the lateral canal lacks the short upward bend at the level of the 

 second dorsal fin which is characteristic in Mustelus. The 



