No. I.] THE VERTEBRATE EAR. 173 



that point strongly to the conclusion that the sense organ at 

 the time of invagination zvas a double one derived from two 

 sources. 



Later on, without, however, dividing completely, these two 

 portions of the sensory apparatus give off each a bud which 

 remains in structural continuity with the parent organ, but 

 which comes to lie in a pocket formed by the pushing out of 

 the anterior median and posterior median portions, respectively, 

 of the utriculo-saccular cavity. These two buds are the sense 

 organs of the recessus utriculi and lagenar pocket, and in this 

 condition the Petromyzon ear remains throughout life, so far as 

 these sense organs are concerned. The nerve supply to these 

 sense organs is derived from the two branches, the ramus an- 

 terior or utriculi and the ramus posterior or sacculi. These 

 nerves seek two widely separated brain centres, the ramus utric- 

 uli running forwards and the ramus sacculi backwards, for their 

 central connections. 



The commissural chamber is of course only a portion of the 

 semicircular canals. Seen from above, the canals run obliquely 

 from the middle of the inner edge of the vesicle to its outer 

 corners, with but very slight curvature. In what manner and 

 at what time the sense organ makes its appearance here is 

 totally unknown, and there is urgent need of information on 

 the ontogeny of this organ and its nerve. 



The Development of the Ear in Elasmoh^anchs. 



Balfour (1876, 15) was the first author who began investigations 

 on the ear of the Elasmobranch type, but his observations are 

 very few in number and do not extend over any but the earliest 

 stages of ontogeny. He closes his remarks on the embryology 

 of the ear by observing {loc. cit. p. 188) that "with reference 

 to the development of the organ of hearing, I have very little 

 to say. Opposite the interval between the seventh and glosso- 

 pharyngeal nerves, the external epiblast becomes thickened and 

 eventually involuted as a vesicle which remains, however, in 

 communication with the exterior by a narrow duct. Towards 

 the close of stage K, the auditory sac presents three protuber- 

 ances, one pointing forwards, a second backwards, and a third 

 outwards. These are respectively the rudiments of the anterior 



