24 ANATOMICAL AND PHYSIOLOGICAL STUDIES ON 



The outer hair cells become higher and wider; they are slightly 

 inclined inward in the upper turn. On passing towards the 

 basal turn the inclination inward increases, and in the basal 

 turn it is most oblique, almost at 45, to the plane of the basilar 

 membrane. In figure 4 the inclination of these cells is only 

 slight. 



Deiters' and Hensen's cells are not well developed; the con- 

 ditions are as in the former stage. 



The plane of the surface of the lesser epithelial ridge is inti- 

 mately related to the development of the outer hair cells and 

 Deiters' cells, and as the latter are in an undeveloped condition, 

 it runs nearly parallel to the plane of the membrana basilaris, 

 sometimes dipping outward. 



The membrana basilaris seems to be much longer; its com- 

 position is about the same as that in the one-day rat, only the 

 thickness is somewhat decreased, owing to the reduction of the 

 rows of cells in the tympanic layer. 



The membrana tectoria grows in breadth and thickness, 

 covering very closely the inner portion of the tympanic wall 

 and connects outwards with Deiters' and Hensen's cells by 

 slender fibrous processes the so-called outer marginal zone. 

 The hairs of the cells stand between these processes, but have 

 no connection with them. 



The vas spirale does not suffer reduction. 



At six days (fig. 6) the development of the cochlea has pro- 

 ceeded futher. The radial breadth of the tympanic wall has 

 increased. Thus we find the tympanic wall, especially its inner 

 portion, increasing towards the apex, chiefly owing to the aug- 

 mentation of the radial breadth of the greater ridge. In this a 

 remarkable change is to be seen. In the basal turn the long 

 slender cells disappear in the inner part of the greater ridge, 

 and instead of them there are found cylindrical cells with oval 

 nuclei near their bases. 



The height of these cells increases gradually to the level of the 

 surface of the inner hah- cell; their upper surface is here in contact 

 with the membrana tectoria. Thus a space appears between 

 the cylindrical epithelium and the membrane the sulcus 



