CEANIAL NERVES AND SENSE ORGANS OF FISHES. 239 



Briefly stated, the lateral line nerves, from being at first 

 considered metameric in character, and formed by the 

 dorsal branches of the various cranial nerves, have now 

 been proved to be entirely independent, and absolutely 

 unconnected with any of the cranial nerves. From an 

 anatomical point of view the lateral nerves are altogether 

 special modifications that have advanced pari passu with 

 the spreading of the organs which they supply, and are 

 just as independent and distinctive as the sense organs 

 themselves. They have their own centre in the bram, 

 the tuberculum acusticum, their own brain lobe, the 

 " lobus trigemini," and their own characteristic nerve 

 fibres, which are larger than any other nerve fibres known. 

 We have a precisely parallel case in the electric battery, 

 electric nerves, and electric lobe of the Torpedo. So far, 

 then, we have not been dealing with nerves which it is 

 possible to arrange in a definite number of pairs or portions 

 of pairs, but with a well-defined single system, whose 

 origin in the brain and ultimate course is a matter of 

 physiological convenience only, and has no relation to 

 any formal plan. 



In the meantime the embryological evidence was slowly 

 veering round so as to fall into line with the anatomical 

 position. This movement was initiated by H. V. Wilson 

 and Mitrophanow, who showed that the auditory organ 

 and lateral line organs proper arose from a common 

 sensory anlage, the differentiation of which, backwards, 

 forwards, and internally, produced the two series of sense 

 organs in question. Here was another proof of the 

 unity of the lateral organs and of their original limited 

 distribution, as well as confirmation of the collateral 

 question of the derivation of the auditory organ from 

 lateral sense organs so conclusively put by Ayers. 

 Further, there is more than a suspicion that ernbry- 



