124 ARRANGEMENT AND STRUCTURE OF SUSTENTACULAR CELLS 



THIRD STAGE OF DEVELOPMENT. 



This is characterized by the peculiar arrangement of the cells of Deiters. 

 Instead of being situated between the interstices of the rows of outer acoustic 

 elements, the body of each sustentacular cell lies just beneath that of its respective 

 supported hair-cell, so that each acoustic element of the first, second, and third rows 

 is supported respectively by the body of a cell of Deiters of the first, second, and 

 third row, as shown in figures 8, 9, 11, and 17 (d 1 , d 11 , d 111 , and oh 1 , oh 11 , oh 111 ). In other 

 words, the axes of the nucleated columns of hair-cells, hence of each individual hair- 

 cell, blends more or less (figs. 8 and 17), or even exactly (figs. 9 and 11), with thatof 

 the nucleated columns of supporting elements that is, of the cells of Deiters. 



In order to obtain a true picture of the mechanical factors which take part in 

 the final arrangement of the sustentacular elements it is only necessary to examine 

 and compare figures 14, 15, and 16, especially the latter, representing the period 

 of rapid elongation of the body of the Deiters cell. In figure 14 the supporting 

 segments (d 1 , d 11 ) embrace one half of the lower pole of each neighboring hair-cell ; in 

 figure 16 they show varying connections with these poles which can only be 

 ascribed to their varying capacity for extension. Owing to the peculiar disposition 

 of the elements of Deiters and the obliquity of their axes, the elongation of the cell 

 body and its segment of support is possible in one direction only i. e., towards the 

 pole of the hair-cell which will eventually be supported by it. Indeed, on lengthen- 

 ing out the segment, d 1 , impinges upon oh", and from this pressure the inclination 

 of the latter may be more pronounced; but the extension of d 1 proceeds towards and 

 around the outer part of the lower pole of oh 1 , so that ultimately the axes of the 

 acoustic elements of the first row will blend with those of the cell bodies of the sus- 

 centacular elements of the first row (figs. 8 and 17, oh l , d 1 ), and the axes of the hair 

 tells of the second and third rows will blend respectively with those of the cell bodies 

 of the sustentacular elements of the second and third rows (figs. 8 and 9, oh", d' 1 ; 

 oh 1 ' 1 , d 111 ) . It may be mentioned that during this process of shifting, the spiral nerve 

 bundles, originally situated outside their respective rows of outer hair-cells (fig. 3, 

 N 1 ", N lv , N v ) are pushed inward, along with the neighboring supporting elements, 

 and now occupy the interstice inside of these sensory rows (fig. 9, N lu , N iv , N v ). 



The upper portion of the body of the Deiters cell that is, the apex of the 

 segment of support, extending around the lower pole of the hair-cell becomes 

 converted into a cup-like depression which surrounds and enwraps the deeper cyto- 

 plasmic part of the acoustic element, particularly at its lower and outer portion, its 

 inner portion for a time remaining free. Whereas in figure 9 this depression is still 

 inconspicuous, in figure 1 1 (owing to a considerable enlargement of the segment of 

 support) it is seen to be deeper and its margins separate the inclosed pole of the hair- 

 cells from the neighboring elements except those situated inside of the pole. In a 

 more advanced stage this denuded part of the hair-cell will in turn be surrounded by 

 the expanding segment of support. 



During these transformations there appears within the cell of Deiters a part of 

 its apparatus of support, originally consisting of a bundle of fibrils, or a fibrillated 



