No. I.] THE VERTEBRATE EAR. 53 



As soon as these two ridges are formed, they are separated by 

 a shallow groove which really forms a part of the sulcus spiralis 

 internus in the adult condition, but in this sense only. The 

 Sauropsid sulcus (internus .''), as I shall call this groove, sepa- 

 rates the two ridges, and its deepest part lies over the boundary 

 line between the two ridges. Now since the large ridge disap- 

 pears during postembryonic development in all mammals yet 

 studied, it follows that at least half of the Sauropsid sulcus is 

 taken up in and forms a part of the space left vacant by the 

 large epithelial ridge, which is the sulcus spiralis internus. For 

 a long time, relatively, the two ridges grow but little, but sud- 

 denly they both receive an impetus and begin increasing in size, 

 the period in their ontogeny having arrived when this phyloge- 

 netic impulse must be reproduced. The larger ridge grows most 

 rapidly under this stimulus, and this may be due to the more 

 direct connection with the nervous centres controlling the organ. 

 From their first appearance there is to be seen over them, 

 covering their apices, a delicate whitish sheet of substance, sup- 

 posed by Kolliker and others to be a gelatinous excretion from 

 the cells themselves, but which more careful examination of 

 properly prepared material shows to be composed of a forest of 

 small hairs growing out of the tops of the cells, which in this 

 manner are shown to be hair-bearing sensory cells, since, as 

 mentioned above, they are connected with the fibres of the 

 cochlear nerve. 



The large epithelial ridge grows in two ways : by increase in 

 the height of the cells, and by increase in its breadth, due to 

 the division of the cells. As the cells increase in size, the hairs 

 grow longer and apparently more rapidly than the cells, for they 

 finally surpass them in length. Having reached its complete 

 development, the large epithelial ridge remains for a time un- 

 changed and in this condition is known as the Organon Kollikeri, 

 this name having been applied to it by Hensen in honor of 

 its first describer, A. von Kolliker. Kolliker's account of this 

 organ is by no means complete, since he failed to make out of 

 the covering membrane or mass anything more than a nearly 

 homogeneous mass of gelatinous substance, which he supposed 

 was simply the increased exudation noticed by him in the earlier 

 stages. The significance of this structure was far from his 

 mind. 



