No. I.] THE VERTEBRATE EAR. 27 



the morphological value of the greater portion of the so-called 

 maada utriculi, the remaining part being the parent organ. 

 Although arising by the division of the utricular sense organ 

 and at first having the same general form and mural relations, 

 the cristas soon begin a growth which makes them quite distinct 

 from these latter. This process of growth is caused by the 

 peculiar mode of development of the ampullae and by the man- 

 ner in which the nerve branch enters the wall to distribute its 

 fibres to the sensory cells. The ventral ampullar wall is pushed 

 upwards into the ampullar cavity in the form of a transverse 

 ridge or crest, in connection with the ingrowth of the nerve. 

 The hair cells cover this crest, and are consequently most ad- 

 vantageously placed for the reception of wave motions of the 

 endolymph. The canal sense organs of the surface of the body 

 are in many species located on crests or elevations in the man- 

 ner just described. The conclusion I wish to draw from the 

 facts recited is that the cristcE aciisticcE of the ampullae retain 

 their original form, while the maculae have lost the tendency to 

 assume the crest shape, i.e. the tendency to acquire an elevated 

 position above the surrounding adjacent surface. In Myxine 

 and Petromyzon the so-called macula acustica communis pos- 

 sesses more than usual interest for the morphologist from the 

 fact, first noted by Retzius, that, owing to its position within 

 the ear cavity, and also to the nerve branches which supply it, 

 the sensory surface is with propriety spoken of as composed of 

 several sense organs. Retzius was led by the nerve distribution 

 to think that perhaps the posterior branch of the anterior root 

 was the homologue of the ramus utriculi, and that the anterior 

 branch of the three branches of the posterior root was the 

 homologue of the ramus sacculi, while the whole of the third 

 branch was the ramus ampullae posterioris, the second branch 

 being the ramulus lagenae. The table given in connection with 

 the account of the distribution of the auditory nerve in Myxine 

 explains more fully my classification of the nerve branches, and 

 particularizes the differences between it and Retzius' classifi- 

 cation. 



Corti's organ, as such, appears for the first time among the 

 mammalia, and it progresses gradually in its complication 

 toward the higher forms. It is derived from the so-called 

 lagena of the Ichthyopsoid and of the Sauropsoid forms lower 



