32 SYDNEY EVANS JOHNSON 



acanthias. The earliest stages of Miistelus ai-ailable and in 

 usable histological condition were 40 mm. in length. In these 

 the head canals and their nerves are already formed. The 

 lateral canal reaches to within 3 or 4 mm. of the caudal extrem- 

 ity and for a few millimeters in this region it has not yet closed, 

 but is still in the form of an open groove. 



The closure of the canal. There are no important differences 

 in the method of canal formation in Mustelus and Squalus. It 

 is essentially a process of the formation of a tube from an in- 

 vaginated groove, though this fact is considerably obscured by 

 irregularities due to unequal growth. As the lateral surface 

 thickening grows posteriorly a groove appears on its surface and 

 extends from the anterior end, where it first appears, posteriorly, 

 lagging considerably behind the growing end of the thickening. 

 As growth continues the gxoove becomes deeper and longer. At 

 the same time the sensor}^ epithelium is differentiating into a 

 thickened ridge. Over this ridge the lips of the groove approach 

 each other in short sections. Between the sections openings 

 remain which become the canal tubules of the adult. The 

 lips opposite each other fuse first at their central points so that 

 the openings between the points of fu.sion appear, in the early 

 stages of fusion, as longitudinal slits. In a specimen 55 mm. 

 long the canal is closed to within a few millimeters of the caudal 

 extremity. The surface openings near the end of the closed 

 part of the canal, however, are still in the form of elongated 

 slits. Passing forward the slits become gradually shorter until, 

 below the second dorsal fin, they are minute, circular openings, 

 which is their normal condition in the adult (fig. 23). 



For a considerable length of time after the lateral canal is 

 formed its lumen remains very small, in some cases appearing to 

 be almost obliterated. Careful examination, however, always 

 reveals its presence, although the inner walls of the canal may 

 be nearly in contact with each other (figs. 24, 25). In these 

 early stages the lumen of the canal must have been overlooked 

 by Balfour as he held the opinion that it was formed by a lumen 

 appearing in a solid cord of cells. 



