BRANCHIAL SENSE ORGANS IN ICHTHYOPSIDA. 11h 
in so far as it describes the origin from the skin of the so-called 
lateral nerve, and in this point it differs from Balfour’s account.! 
It is, as Semper stated, very easy in Elasmobranchs, though 
by no means so in Teleostei, to follow the whole development 
of the lateral line and nerve. 
In horizontal longitudinal sections the whole process is 
obvious enough, and I can fully endorse Van Wijhe in the 
opinion that Balfour would have had no doubt about the matter 
had he studied the point with horizontal sections instead of with 
transverse ones. The question of the direction of sections is 
here a vital one. In (fig. 39, vg. gl.) the compound vagus ganglion 
is represented as fused with the skin, and the lateral line, J. /., 
has commenced to grow backwards. 
It is an interesting, and by no means an unimportant point, 
that the lateral line increases in length not by the actual con- 
version of the epiblast cells behind the growing point of the 
line into sensory cells 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 indifferent epiblastic cells behind it (fig. 40). 
These indifferent epiblastic cells (figs. 40 and 411, z. e.) are actually 
thrust aside and probably lost along the whole course 
of the “lateral line” and concomitantly with 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 Elas- 
mobranchs. 
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 
' Balfour, ‘ Elasmobranch Fishes,’ p. 141. 
