184 JOHN BEABD. 



similar to those already present in the line, but that there is an actual 

 growth backwards of the lateral line itself (figs. 40 and 41). That is, 

 the sensory cells which compose the rudiments of the " line," and 

 ■which anteriorly give rise to the compound vagus ganglion {vg. 2, 3, 

 and 4), repeatedly and rapidly divide, and in such a manner that 

 the " line" is increased in length and pushes its way between the indif- 

 ferent epiblastic cells behind it (fig. 40). These indifferent epiblastic 

 cells (figs. 40 and 41, i.e.) are actually thrust aside and probably lost 

 along the ivliole course of the " lateral line " and concomitantly ivith its 

 growth. 



Part of the epiblast which is cast off is figured in figs. 40, 41, i.e. 

 It is possibly this temporary epiblast seen in transverse section which 

 led to Balfour's view of a special origin of the canals of the sense organs 

 in the trunk of Elasmobranchs. 



As in other cases the nerve of the sense organs, the so-called lateral 

 nerve, is formed from the deeper portion of the sensory thickening. 

 This mode of origin of the lateral nerve, was first described by Semper, 

 and afterwards more fully by Van Wijhe in Elasmobranchs. 



The point is far easier to determine here than in the case of other 

 supra-branchial nerves ; indeed, it attracts the eye with startling dis- 

 tinctness in horizontal longitudinal sections of embryos of the proper 

 age. The nerve is formed as the sensory thickening grows backwards 

 along the body. It is well shown in figs. 40 and 41, l.n., and can be 

 traced from the vagus ganglion (vg.gl.) backwards along the thickening, 

 gradually becoming thinner and less differentiated until finally it 

 ceases in the cells of the sensory thickening. 



That here there is no actual growth backwards of the nerve is 

 obvious enough, for when the development has taken place for some 

 length, then near the ganglion the nerve is fibrillar and has few nuclei, 

 these latter increasing as the nerve proceeds backwards, and the fibres 

 becoming, pari 2>assu, fewer, and ending gradually in the protoplasm 

 of the sensory thickening. 



Where the compound vagus ganglion (vg.gl. 2, 3, 4) separates from 

 the skin (fig. 36) it is easily seen that above each of the three 

 branchial clefts fibres are given off from the separating ganglion to 

 the sensory thickening. In fact, each of the elementary nerves making 

 up the vagus compound, viz. vg. 2 and 3, and the intestinal branch, 

 vg. 4 and 5, takes part in the formation of the so-called " lateral line." 

 In other words, the lateral line is made up of supra-branchial branches 



