578 LOCK [Vol. XI. 



branchiae. As Mitrophanow has pointed out, this thickened 

 patch of epithelium is composite in character ; it embraces not 

 only the epithelium for the formation of the auditory plate, but 

 that to form the so-called branchial sense-organs, and also the 

 beginning of the lateral line. I had noted this relationship in 

 Squalus before seeing Mitrophanow's paper, and I find no com- 

 parison that my results correspond — so far as the earlier stages 

 are concerned — very closely with his. He has, however, car- 

 ried his studies considerably farther, and has shown how the 

 different branches of the lateral-line system arise. 



The general relationship noted between the auditory plate, 

 the branchial sense-organs, and the lateral line is very similar 

 to what Wilson found in sea bass. In that form the connec- 

 tion between ear, branchial organs, and lateral line is evidently 

 more clear on account of the presence of a distinct sensory 

 furrow and the way in which the separation into three parts 

 takes place. 



The general thickening in the sharks is not so well circum- 

 scribed. The fact that these different organs proceed from a 

 common epithelial thickening indicates relationship of a funda- 

 mental kind. After separation the ear gives further evidence 

 of its relationship with the lateral organs by preserving its 

 canal (endolymphatic duct) connection with the exterior and 

 developing in a manner that is characteristic of the canal organs 

 of the lateral line. 



Mitraphanow departs from the usual point of view that the 

 organs of the lateral line are metameric, and in that particular, 

 I think, I should be inclined to follow him. 



The auditory plate is at first differentiated from the general 

 thickening, and its epithelium then becomes gradually rounded 

 up into a circular area, that is depressed in the centre — the 

 auditory saucer. This structure sometimes covers the space 

 of three neuromeres, and I have one surface preparation in 

 which the outer epithelium shows three bars running vertically 

 across it; these bars correspond with the underlying neuro- 

 meres. 



We may interpret these superficial bars either as being 

 moulded upon the underlying neural segments, or as being 



