120 ARRANGEMENT AND STRUCTURE OF SUSTENTACULAR CELLS 



the third row of acoustic elements. The basal, nucleated portion of these support- 

 ing cells follows the shifting of their more superficial part, and each of the three 

 originally mixed outer spiral rows divides into two distinct rows : One, purely sensory, 

 which does not reach the basilar membrane; the other, purely sustentacular, which 

 is continuous throughout the thickness of the epithelium. Of all these elements 

 the apices alone maintain their primitive position. At the same time other import- 

 ant changes are occurring. The basal, nucleated portion of the cells of Deiters 

 undergoes a slight enlargement, while the increase in the superficial cytoplasmic 

 part is inconspicuous; as a result of mutual compression between the neighboring 

 hair-cells it is still flattened out in a spiral direction and thereby reduced in size. 



In this respect the outer pillars differ from the outer supporting cells. Like 

 the inner pillars they enlarge rapidly, and three parts, without marked outlines, 

 become distinguishable; i. e., a nucleated basal portion, or foot, which is the largest; 

 an intermediate, cytoplasmic portion, of medium size, which tapers off to a more 

 superficial portion, the latter being the smallest (fig. 5). At its foot the outer pillar- 

 cell assumes the form of a four-sided, and in its more superficial part a five-sided 

 pyramid (figs. 4 and 5), the basal part of which is flattened out in a radial direction 

 (fig. 5, op). A similar but more pronounced increase in size and a subdivision into 

 three segments are noticeable in the inner rods of Corti (figs. 4 and 5, ip), the foot 

 and nucleus being markedly flattened out in a radial direction. Moreover, the 

 foot extends considerably toward the foramina nervina, seemingly repelling the 

 bases of the neighboring inner cells. This process of extension has been pointed out 

 by many observers Hensen, Boettcher, Middendorp, Retzius and others. No 

 true shifting of the base of any supporting cell, however, occurs. The expansion 

 of the foot of the inner pillars inward and of the foot of the outer pillars outward is 

 accompanied by a corresponding extension of the subjacent basilar membrane. 

 This has already been emphazised by Vernieuwe and others, and should be attrib- 

 uted to abundant nutriment from the neighboring vas spirale. This elongation of 

 the base of the inner pillar-cell is accompanied by an alteration in the inclination 

 of the inner pillar itself, and causes likewise an alteration in the inclination of the 

 inner supporting and hair-cells. The degree of slant of these cells is difficult to 

 determine accurately, although it is quite marked. Indeed, sections tangential to 

 the surface of the organ of Corti, in the earliest stages of development, represent 

 cross-sections through all of its elements (figs. 2, 3 and 4). When the inner pillars 

 extend toward the foramina nervina such sections cut transversely the components 

 of the outer part of the spiral organ (figs. 6, 7, 8 and 9), while the inner pillars, the 

 inner hair and supporting cells are frequently cut along their length. This can be 

 explained only by their marked inclination outward and towards the surface, and 

 the inclination of the outer hair and supporting elements in another direction, so 

 that the axis of the inner supporting cells (fig. 16, is') meets that of the outer at 

 nearly right angles. 



During the second stage of development of the organ of Corti, and soon after 

 the protoplasmic portions of the cells of Deiters reach their neighboring interstice, 

 another remarkable shifting occurs -a shifting in the spiral direction of the organ, 



