54 AYERS. [Vol. VI. 



The little epithelial ridge grows more slowly, and at the time 

 the large ridge has reached the height of its development, it is 

 still in a very immature state, though the structures which serve 

 to distinguish it and make it peculiarly a mammalian organ are 

 marked out so definitely that no parts are added to the organ as 

 it now exists. From this time on, it is evidently the organ of 

 Corti, though about this homology there exists in the literature 

 a great amount of confusion. Retzius correctly recognizes the 

 ontogenetic relation of the adult organ of Corti to the small 

 epithelial ridge, but he says the organ of Corti is the homologue 

 of the macula acustica papillae basilaris, which it never is. Even 

 among the higher Sauropsida (saurians and birds) there is such 

 a differentiation of parts as to necessitate our recognition of the 

 fact that the sensory plate of the papilla acustica basilaris of 

 these forms cannot be considered the homologue of the simpler 

 plate in the lower forms. Likewise, when we pass by the un- 

 known forms between the Sauropsida and the Mammalia we 

 find ourselves face to face with an entirely new organ that has, 

 at most, only the slightest indications of its beginnings in the 

 ear of the Alligator, the intervening stages of development so 

 far as at present known having been annihilated with their 

 possessors, the Sauropsid ancestors of the Mammalia. 



The causes which have led up to the annihilation of the large 

 epithelial ridge during ontogenetic process as a part of the phy- 

 logenetic advancement of the Mammalia are very obscure as 

 yet. For the present, suffice it to say that such is the fact, 

 easy of observation, and that connected with this annihilation 

 of the Sauropsid organ is the rapid advancement and perfection 

 of the mammalian organ of Corti. The Sauropsid organ at the 

 height of its development takes more of the nerve supply from 

 the cochlear nerve than does the organ of Corti ; but as the latter 

 grows and the former fades, this condition is reversed, until the 

 cochlear nerve gives off fibres to the organ of Corti alone. What 

 becomes of the fibres supplied to the Sauropsid organ .'' Do they 

 simply disappear through resorption } I have observed that the 

 bundle gradually grows smaller, but I am unable to detail the 

 process of disappearance. 



The Sauropsid organ disappears through a process of resorp- 

 tion ; that is, first of all, through a loss of its nutritive supply, 

 and then by a gradual decrease in size of the cells until only a 



