138 AVERS. [Vol. VI. 



classed by raorphologists as two distinct cranial nerves. As I 

 have already stated, it is generally believed that these canal 

 organs subserve a function akin to audition. 



In the case of organs 16 and 17, their centrally distinct nerves 

 run by separate paths and end peripherally in separate organs, 

 having different pedigrees. The two organs, however, are in- 

 closed in a common canal, which communicates with the surface 

 by a single pore. Let us suppose these two organs with their 

 incipient canals sunk below the surface of the head in a common 

 depression, such as AUis describes for the organs themselves, 

 early in development ; the thickened plate of ectoderm out 

 of which they were forming would represent not only the 

 sensory epithelium, but also the epithelial lining of the canals, 

 which would normally form were they to remain on the surface 

 of the body. These canals would be represented by some 

 growth after thus sinking below the surface. Thus, in the act 

 of sinking, a common canal would arise, vesicular in shape, on 

 account of the mechanical conditions of the deep involution. 

 This common canal would open on the surface of the body by 

 a pore, and this region of the canal, as the sense organs sank 

 deeper into the head, would be drawn out into a tube. In thus 

 going further below the surface of the head than usual, so far 

 indeed that the connection with the surface canals would be 

 broken, the sense organs would jiot lose their tendency to mul- 

 tiply as they normally multiplied on the surface of the body, 

 and as they divided, their nerves would also be broken up into 

 as many peripheral branchlets as there were sense organs. The 

 increase in number of the organs, and especially their increase 

 in functional importance, would lead to an alteration of the 

 relation of the nerve branches supplying them. The two pri- 

 mary branches would become more and more independent of 

 their parent trunks, the VII and IX, respectively, and would, 

 on account of the compactness of the territory supplied by 

 them, be drawn more and more together, until they apparently 

 formed a single large nerve passing directly from the brain to 

 the organ complex supplied by them. 



This process would influence the central origin, but in much 

 less degree. However, as functional differentiation progressed, 

 there would be a greater and greater separation from the pa- 

 rental ganglionic centres of those cells receiving the now special- 



