BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 183 



st.v.). Here, as in the glossopharyngeal, the supra-branchial branch 

 has a dorso-anterior direction. 



Vagus I. also fits into the schema very well. It is formed just in 

 the way described in the schema, has the same relation to a cleft, 

 develops a primitive branchial sense organ and associated ganglion, 

 &c. In fact, its development might have been taken in giving the 

 schema. 



For the rest of the vagus there is only one ganglionic mass, and one 

 long, broadish thickening with which the ganglionic mass is associated. 



When the common nerve rudiment grows from the neural ridge and 

 fuses with the epiblast, at the point of fusion the ganglionic mass is 

 proliferated, probably entirely from the skin. From the ganglionic 

 mass branches are sent off along the posterior sides of each of the 

 three last clefts to the musculature of the clefts. They are the 

 post-branchial branches, and are not developed from the skin. The 

 last of the three is the so-called intestinal branch of the vagus. 

 Along with the separation of the ganglion from the skin, the sensory 

 thickening begins to grow backwards along the lateral surface of the 

 trunk (fig. 39). This thickening is the rudiment of the so-called 

 lateral line, The description of its development to be given here is 

 in the main identical with that given by Van Wijhe. 1 It agrees with 

 Gotte's 2 and Semper's 3 researches 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. 4 It is, as Semper stated, very easy in Elas- 

 mobranchs, 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 Lpiestion 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, LI., 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 conversion of the 

 epiblast cells behind the growing point of the line into sensory cells 



1 Op. cit., pp. 34, 35. 



2 Gotte, ' Entwickelungsgesch. d. Unke,' p. 672. 



3 Op. cit., p. 256. 



4 Balfour, ' Elasmobranch Fishes,' p. 141, 



