No. I.] THE VERTEBRATE EAR. 3 



or incipient canal, and he equally failed to see the value of 

 the semicircular canals as evidence of the descent of the 

 auditory organ; for he continues, loc. cit., "the semicircular 

 canals, etc. (meaning thereby, I infer, the utriculus, sacculus, 

 cochlea, and the other parts of the so-called labyrinth) are 

 clearly secondary complications ; for in every embryo the audi- 

 tory organ is at first a simple sac, shut off from the epi- 

 dermis ; of which sac a portion of the inner wall consists of 

 two layers of modified epiblastic cells, connected by a dorsal 

 sensory branch of a segmental nerve with the brain." As is 

 well known, the ear is usually inclosed within the head before 

 the surface canal organs are formed ; so that, excepting a few 

 forms, the direct connection of the auditory organ with the 

 canal sense organs which remain on the surface of the body is 

 not an ontogenetic occurrence. 



In sharks, for example, the ear appears long before the 

 other sense organs of the surface of the body, and has sunk 

 below the surface, divided, and begun the production of semi- 

 circular canals by the time the organs that are to remain on the 

 surface of the body are apparent. Mitrophanow has recently 

 worked on the development of the lateral line organs in Elasmo- 

 branchs. On what species I do not know, since, unfortunately, 

 I have been unable to consult his paper published in Russian. 

 My own observations begun in the summer of 1889 at the 

 Marine Biological Laboratory, Wood's Holl, Mass., on embryos 

 of the Smooth and the Spiny Dogfish {Galens canis and Acan- 

 thias vulgaris) and an undetermined species of Batoid, probably 

 Torpedo occidentalis, leave no doubt that the ear is derived from 

 the sense organs of the lateral line group ; for in these species 

 the ear retains its canal connection with the so-called aural 

 group or line of surface organs long after the canal organs and 

 their canals have become well developed. 



My observations would perhaps have been fruitless of the 

 main result had I not at that time received a copy of Allis's 

 paper on the development of the lateral line organs of the 

 Ganoid Dogfish. 



While reading his account of the manner in which the sur- 

 face canals are formed in that fish, it was recalled to mind that 

 Van Noorden had described the formation of ampullae and ear 



