AND HAIR-CELLS IN THE DEVELOPING ORGAN OF CORTI. 129 



gone a peculiar differentiation. Around the nuclei of the deeper group (ad lv ) it 

 appears clear or vacuolated like that of the cells of Deiters (d iu ) ; around the nuclei 

 of the superficial group (aoh' v ) it is dark and granular, like that of the hair-cells 

 (oh m ). From this it is plainly evident that the protoplasm of these two cells (ad iv 

 and aoh' v ) is beginning to differentiate respectively into supporting and sensory 

 elements; but their apices, as can be seen on close examination of serial sections, are 

 represented by a mosaic of undifferentiated polygons similar in character to those 

 of the true neighboring cells of Hensen. In reality, therefore, these cells should 

 be regarded respectively as aborted supporting (ad iv ) and sensory (aoh' v ) elements 

 of a fourth spiral row, which in man, especially, and also in some animals, become 

 completely developed. In most animals, however, they retain their primitive 

 character of simple, lining epithelial, or atrophied acoustic elements. Quite excep- 

 tionally a fifth spiral row of aborted hair-cells may be observed (fig. 7, aoh v ) external 

 to the fourth (aoh iv ). 



The atrophied acoustic elements are characterized by their arrangement into 

 a spiral row situated external to the third row of cells of Deiters, and by the fact 

 that their lower ends never reach the membrana basilaris (fig. 11, aoh lv ). The apex 

 of the cell is represented by a large polygonal field containing a diplosome (fig. 13, 

 aoh iv ) , not unlike the apex of the true cell of Hensen. The atrophied hair-cells of the 

 fourth outer row persist during the course of development and are illustrated in 

 sections tangential to the surface in figures 8, 9, 17, and especially in figure 11 (aoh iv ), 

 and also in vertical, radial sections in figures 15 and 16 (aoh lv ). The atrophy 

 continues, apparently, for in adult animals these cells are much shorter than in 

 embryonic stages, although they are never absent. 



As regards the atrophied cells of Deiters (figs. 2 and 5, ad iv ) these belong to the 

 spiral row of atrophied hair-cells and, contrary to the normally developed susten- 

 tacular elements, do not reach the surface of the epithelium. As seen in vertical, 

 spiral sections (fig. 21, ad iv ), they alternate with the more superficial elements 

 (aoh iv ) without passing between them. The phalanx process is absent, so that at 

 this point the epithelium is formed of two strata of atrophied cells. Further investi- 

 gations will have to be made before the arrangement of the atrophied sustentacular 

 cells in more advanced stages of development can be determined. The atrophy of 

 these elements seems to be accompanied and caused by the lack of corresponding 

 nerve fibers. 



MITOCHONDRIA AND OTHER STRUCTURES IN HAIR AND SUPPORTING ELEMENTS. 



HAIR-CELLS. 



In the earliest stages of development of the organ of Corti (fig. 1) the cytoplasm 

 of the acoustic elements is densely packed with mitochondria, which are lined up 

 into granular chondriomites, or, as is usually the case, fused together into uniform 

 filaments or chondrioconts. These are seen to run parallel with the axis of the 

 cell and produce a longitudinal striation ; in more advanced stages of development 

 they still constitute the bulk of the protoplasm (figs. 19 and 20, oh ; , oh", oh'" ; fig. 9, ih) . 



