194 



AYERS. [Vol. VI. 



malian intuitions. The cochlear organ of the Alligator is not a 

 genuine organ of Corti, but is the cochlear organ of the saurian 

 type of reptilian ear developed along the line of histological 

 differentiation laid down in the simpler members of the group. 

 By this I do not wish to say that the saurian cochlear organ is 

 the strict homologue of the macula acustica or papilla basilaris 

 of the Amphibia and lower groups of reptiles. 



The inner hair cells are well developed and are already sepa- 

 rated from the outer hair cells, not only by the developing 

 pillars of Corti, but also histologically by the appearance of 

 individual characters which persist and are increased in later 

 life. Some of these characters are, briefly, a different shape of 

 cell body, different length of the hairs, a different relation to 

 the nerve fibres and supporting cells, and to the basilar mem- 

 brane and arch of Corti. 



The main process met with in the transformation of the epi- 

 thelium of the floor of the embryonic cochlear tube consists in 

 the rapid increase in size and number of the cells along certain 

 lines parallel with the long axis of the cochlea. There are 

 two such ridges which need our special attention : I. The large 

 epithelial ridge ; II. The small epithelial ridge. 



T\\& first, or large, ridge begins near the habenula perforata, 

 and extends outwards, covering about two-thirds of the basilar 

 membrane in radial direction ; on its outer slope it passes im- 

 mediately into the 



Second, or the small, ridge which extends outward, i.e. in 

 transverse direction to about the beginning of the zona pectinata. 

 It equals in length the large ridge, but is inferior in height and 

 breadth. 



According to Boettcher the whole of Corti's organ is devel- 

 oped from the small epithelial fold, while the large ridge gradu- 

 ally undergoes reduction until it disappears as a structure of the 

 cochlear canal ; its cells form the lining of the sulcus spiralis in- 

 ternus. The first cell of the small ridge develops into the inner 

 hair cell. Boettcher holds that each inner hair cell is composed 

 of tJiree cells. The second cell of the small ridge develops into 

 the pillars of Corti. The third cell of the small ridge develops 

 in the first row of outer hair cells. The fourth cell of the small 

 ridge develops in the second row of outer hair cells. The fifth 

 cell of the small ridge develops in the third row of outer hair 



