BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 187 



thickening, and further, the connection between sense-thickening and 

 nerve is best made out in early stages, and is afterwards not so easy 

 to trace. 



Van Wijhe himself, though he has given a true, accurate, but some- 

 what incomplete account of the development of these supra-branchial 

 branches to the sense organs, cannot be said to have solved the 

 difficulty under discussion. He has rather ignored it, and though 

 possessing the material for its solution, has not mentioned the matter. 

 It is very curious that, although he has figured the fusion of various 

 ganglia with the skin, he has apparently not noticed that the supra- 

 branchial branches grow in the various cases out of the various ganglia 

 so fused, and therefore are in connection with their expropriate ganglia 

 from the first. 



In fact the whole rationale of the formation of supra-branchial 

 nerves is to be seen in the deploying of the branchial sense organs, 

 and in the connection of these organs with the ganglionic centre by 

 longer or shorter conducting fibres — the supra-branchial nerves. 

 Originally the sense organs were restricted to one over each gill-cleft, 

 with an associated ganglion. 1 This increased, and gave rise to two by 

 division, and so on. This is the more certain when we remember that 

 even in late stages, according to Malbranc, 2 the sense organs of 

 Amphibia increase by division. I have myself noticed and recorded 

 this mode of increase in embryonic Teleostei. 3 



It is hardly necessary to repeat that Gegenbaur's view of the com- 

 position of the vagus out of a number of typical posterior roots is 

 quite true. We have seen that it really contains rudiments of at 

 least five such elements in Torpedo. 



It follows from this that the vagus agrees with the schema given in 

 the preceding pages. It is equivalent to and shows the development 

 of, at least four such schematic nerves, True, there is only one supra- 

 branchial branch, 4 the lateral nerve, for all the elements of the vagus 

 except the first. But this is probably secondary, and due to the fusion 



1 Beard, " Segmental Sense Organs and Associated Ganglia," ' Zool. Anz.,' 192, 1885 ; also 

 Froriep, "Ueber Anlagen von Sinnesorganen am Facialis, &c," 'Archiv fur Anat. und 

 Physiol.," 1885. 



2 Malbranc, "Von der Seitenlinie u. ihren Sinnesorganen bei Amphibien," 'Zeit. f. wiss. 

 Zool.' vol. xxvi, 1876. 



3 Beard, " Segmental Sense Organs of Lateral Line," ' Zool. Anzeiger,' Nos. 161, 162, 

 1881. 



1 In Torpedo and many other forms. In other cases the " lateral line " is more compli- 

 cated ; especially is this the case in Amphibia, vide Malbranc, op. cit. 



