GROWTH OF THE INNER EAR OF ALBINO RAT 23 



diminishes at this stage towards the apex, though it is larger 

 in the beginning of the middle turn than in the middle of the 

 basal turn. As will be shown later, these relations are entirely 

 reversed in the adult cochlea. This fact indicates that the 

 cochlea at this stage is very immature. 



In the three-day-old rat the cochlea is much better developed 

 (fig. 5). The radial breadth of the typmanic wall of the ductus 

 cochlearis becomes greater in all the turns, especially in the upper 

 turn; therefore the differences between the radial breadths in 

 each successive turn are smaller than at the earlier stage. There 

 is some change as we pass towards the apex in the relation of 

 the inner and outer portion of the tympanic wall. At the basal 

 turn and the beginning of the middle turn the radial breadth 

 of the outer portion increases greatly, but diminishes again 

 towards the apex. Although the radial breadth of the inner 

 portion increases through all the turns, the proportion of this 

 increase becomes greater towards the apex. As the inner portion 

 is composed of the greater epithelial ridge and of the limbus 

 laminae spiralis, and as the breadth of the latter diminishes 

 towards the apex, the increase of the radial breadth of the inner 

 portion is due to changes in the greater epithelial ridge. 



The heights of the greater epithelial ridge, however, diminishes 

 through the successive turns, becoming less and less from base 

 to apex. Thus in the cochlea at this age it has a small radial 

 breadth and vertical height in the basal turn and a larger radial 

 breadth and height in the upper turns. 



In all the turns the inner hair cell is inclined outwards and 

 lies with its surface forming the outermost part of the greater 

 ridge. The obtuse angle which it helps to make (fig. 5) as a 

 boundary between the greater and lesser ridge in upper turns, 

 vanishes in the basal turn where there is no sharp boundary 

 between the two ridges. 



The pillar cells of Corti develop more and more during this 

 early stage; the radial breadth of their base increases, but as 

 yet there is no space between them. They incline much more 

 outwards than in the earlier stage. The protoplasmic change 

 in the rod progresses, especially in the basal turn, and the head 

 plate of the cell can be seen distinctly. 



