MUSEUM OF COMPARATIVE ZOOLOGY. 59 



a little behind the ear capsule, on the level of the notochord ; and it is 

 formed by the splitting off of some of the cells of the inner layer of the 

 epiblast. From its point of origin, where it is bi'oadest, it grows back- 

 wards along the body. This cord of cells, as Beard calls it, is no doubt 

 what Balfour describes, in Scyllium, as a linear thickening of the mucous 

 layer of the epidermis, or as a linear streak of modified epidermis. This 

 linear sense streak is in Balfour's opinion the primitive structui'e from 

 which the various forms of the line have originated. He says, further, 

 that the thickened streak becomes a canal in Scyllium, not by the fold- 

 ing over of the sides, as in Teleostei, but by the formation of a cavity 

 between the epidermic and the mucous layers of the epiblast, and the 

 subsequent enclosure of this cavity by the modified cells of the mucous 

 layer of the epiblast constituting the lateral line. The cavity appeared 

 first at the hinder end of the organ, and thence extended forwards. 

 After formation the canal gradually recedes from the surface, i*etaining 

 its connection, however, at a series of points corresponding to the seg- 

 ments, points at which the segmental openings are afterward formed. 

 As compared with the open canal on Chimcera, the tubular canal is a 

 secondary form. In regard to the innervation on the trunk by the 

 lateral branch of the vagus, the nerve was found to originate as the 

 other nerves, and, pushing its way backward, to follow the course of 

 the lateral line. Originally the line is supposed to have been restricted 

 to the anterior part of the body, and thence, extending farther and 

 farther backward, it carried with it the lateral branch of the vagus, 

 until ultimately the latter was prolonged far beyond the position it 

 originally occupied. Beard says of this nerve, in Salmo, that it origi- 

 nates far from the epiblast and growing backward approaches the skin 

 so as to lie between the two muscle plates just below the epiblast, never 

 fusing with the lateral line, but always separated from it by the cuticular 

 basement membrane of the epidermis. 



Balfour found the canals of the head and the ducts of the ampullae 

 to be formed from the mucous layer of the epidermis, very much as 

 the lateral line ; but their innervation is effected by simple branches 

 of the fifth and seventh pairs, which reach them in various places 

 without following their courses, unlike the nervus lateralis and the 

 lateral line. 



Primarily the openings at the ends and along the sides of the tubes 

 appear to have been in close relationship with the segments of the body, 

 both in regard to position and number. The relations are still apparent 

 in the numbers and in the positions of the tubules at their points of 



