2i6 AVERS. [Vol. VI. 



between the facial and glossopharyngeal nerves. (For Elasmo- 

 branchs, cf. Beard, VIII, Rep. Scot. Fishery Bd., 1889, PI. X, 

 Fig. 15 ; and for man, His, Arch f. Anat., 1887, p. 376, Fig. 8.) 



26. The ear progresses in its ontogeny according to the plan 

 of development of the lateral line sense organs of the Ichthy- 

 opsida. There as here the appearance or disappearance of parts 

 of the ear chamber are preceded, introduced, and governed by 

 the division or death of the contained sense organs. The facts 

 of development strengthen the conclusions drawn from compar- 

 ative anatomy and show that the Cyclostome ear is the fore- 

 runner and an ancestral stage of the Gnathostome ear, and 

 Retzius's idea that an impassable chasm exists between the ears 

 of the two types is proven to be incorrect. 



27. It is in the highest degree probable that the sense organ 

 which I have discovered in the endolymphatic duct or surface 

 ear canal of Petromyzon's ear exists also in the reptilian ear at 

 least during ontogeny, for its nerve is well developed in the 

 saurians and passes into the zvall of the endolymphatic duct. 



28. Hensen, among others, advocated the idea that the large 

 epithelial ridge of the embryonic cochlea was the organ whose 

 function it was to secrete the membrana tectoria, and most 

 authors since his time have accepted this idea without question, 

 notably Retzius. It has been taken for granted that all of the 

 membrana tectoria of the embryo was converted into that of 

 the adult, but that such a process does not occur is of course 

 perfectly evident when one follows the resorption of the Sau- 

 ropsid organ ; for as the latter disappears, its capillary structures 

 disappear with it, and the adult mammalian hair band is made 

 up of the hairs springing from the smaller epithelial ridge alone. 

 The embryonic membrane of Corti is made up of hairs of both 

 large and small epithelial ridges, while the adult membrane is 

 the product of the small epithelial ridge alone or in terms of 

 adult anatomy; the embryonic "membrane" includes the prod- 

 uct of both the transient Sauropsid organ and the permanent 

 organ of Corti, while the adult "membrane" is of course the 

 persistent portion of the embryonic membrane borne by the 

 cells of the small ridge which constitute the hair cells of 

 the adult organ of Corti. 



