198 MR. F. J. COLE ON THE STRUCTURE AND MORPHOLOGY OF 



that in both these respects (?'. e. histology and embryology) the agreement between the 

 lateral and auditory organs is so remarkable as to admit of only one explanation, and 

 that favourable to the close geuetic relationship of the two series. When, however, 

 we descend to details, and enquire into the phylogeny of the semicircular canals, the way 

 is not so clear. Ayers contends that a lateral line canal may differentiate further after 

 being cvit off from the surface. I have already joined issue with him on this point, and 

 have urged that there is no direct evidence favouring this statement. The lateral canals are 

 jirimitively laid down in the form they are found in the adult, and the only possibility of 

 svich differentiation is perhaps to be found in the blind diverticula of the canals described 

 by Hyrtl, Sappey, Pollard, the writer, and others. Ayers' postulate is by no means 

 impossible, but is at present mere speculation, and not founded on fact. The bearing 

 of this on the question of the semicircular canals is obvious. The latter develop as 

 evaginations of the wall of the auditory sac after the sac has become cut off from the 

 surface *, and is in a condition corresponding to a, lateral line canal. It is true that the 

 lumina of the semicircular canals are pieces cut off from the outside world, and one can 

 admire Ayers' comparison of a semicircular canal with a single segment of a lateral canal 

 still opening on to the surface (= cavity of sac) by its two terminal pores. These con- 

 siderations, and the others advanced by Ayers, do not discount the cardinal fact that the 

 semicircular canals of all Vertebrates develop in a way for which there is no jiarallel in 



r knowledge of the anatomy and development of the lateral line system. At one time 



1 thought that the ear with its semicircular canals was formed by a concentration of certain 

 lateral canals, and that the aqueeductus vestibuli or ductus endolyniphaticus corresponded 

 to a dermal tubule connecting an internal canal with the exterior. Both these contentions 

 are held and ably advocated by Ayers, and the latter undoubtedly holds good. The 

 question that remains is whether the auditory organ has arisen by the diflfereutiatioa of 

 an organ such as an ampulla of Lorenzini, or whether it was formed by the concentration 

 of certain primitive canals of the ancestral Vertebrate. In spite of the wealth of argu- 

 ment employed by Ayers, I cannot pass over the developmental difference described 

 al)ove, and must therefore hold to the former view, which by the way explains why there 

 shoiild be only one aquseductvis vestibuli in the history of the auditory organ. I thei-efore 

 follow Beard in regarding the ancestral auditory organ as represented by the primitive 

 auditory sac of the embryo, and that the semicircular canals arose later in the ancestral 

 history of the organ, and after it had entirely lost connection with the lateral line system. 

 Such a view of course involves the corollary that although the semicircular canals bear 

 many points of resemblance to the sensory canals of the lateral line, yet they must be 

 ren-arded as only homoplastic and not homologous Avith them. Ayers' view, on the other 

 hand, implies that the auditory organ was from the first a complicated structure and 

 possessed semicircvilar canals. 



* [n some of the lower Vertebrates, of course, this never comjihteh/ bai)pens. 



