AND HAIR-CELLS IN THE DEVELOPING ORGAN OF CORTI. 133 



the deeper portions of the cell, where, at the level of the intermediate and basal 

 parts it is converted into a solid latero-medial lamellar strand (figs. 3 and 4, ip) 

 reaching the basilar membrane. In more advanced stages the tube itself becomes 

 transformed into a solid, cylindrical (fig. 21, ip), and later into a flattened, lamellar 

 mitochondrial bundle (fig. 18, ip) which also soon occupies the medial half of the 

 superficial segment of the pillar (fig. 20, ip). This longitudinal, lamellar mito- 

 chondrial bundle, extending throughout the latero-medial portion of the proto- 

 plasm, is illustrated in vertical sections (figs. 15 and 9, ip), and by coalescence of 

 its chondrioconts gives rise to the fibrillar supporting apparatus of this inner pillar 

 (figs. 14, 17, and 8, ip). 



Besides this bundle, the bulk of the inner pillar during the second and third 

 stages of development is formed, at the level of its basal and intermediate segment, 

 of a clear, vacuolated cytoplasma (figs. 4, 5, 14, and 16, ip), occupying its axial and 

 lateral portions. Its apical segment is composed of a compact, almost homogeneous 

 protoplasma (fig. 5, ip) external to (fig. 20, ip) or even surrounding (fig. 18, ip) the 

 supporting bundle. 



The axial mitochondrial strand of the outer pillar-cells (fig. 2, op) also becomes 

 converted into a paraxial or lateral, solid bundle, flattened or lamellar in shape, 

 which originally runs more or less vertically throughout the three segments of the 

 cytoplasm (figs. 4 and 5, op). In more advanced stages, while the apical segment 

 undergoes an enlargement and develops the head of the pillar, the mitochondrial 

 strand becomes obviously interrupted at the level of the junction of the two upper 

 segments. The superposed chondrioconts (fig. 20, op) fuse together and give rise to 

 two distinct fibrillar bundles; a lower bundle, belonging to the basal and interme- 

 diate parts of the pillar (figs. 9, 14, 15, and 16, op), and a superficial one, belonging 

 to the apical segment that is, the fibrillar supporting band connecting the phalanx 

 with the head of the pillar (fig. 13, op). This filamentous band, originally vertical, 

 later runs obliquely towards its phalanx between the superficial cytoplasmic por- 

 tions of two neighboring outer hair-cells (fig. 14, op), its course becoming more hori- 

 zontal as development progresses (figs. 16 and 17, op). Thus, during the second 

 and third stage in the development of the organ of Corti, the outer pillar, at the 

 level of its basal and intermediate segments, is composed of a lateral mitochondrial 

 or fibrillar, lamellar bundle, and an axial and inner, large, vacuolated portion (figs. 

 4, 5, 14, 16, 7, and 9, op), representing at a certain time (fig. 9, op) the bulk of the 

 cytoplasm. At the level of its apical segment the pillar is composed of its oblique 

 or nearly horizontal fibrillar bundle and a compact cytoplasm the bulk of the head. 



It is worthy of note that prolongations from the superficial terminal bars 

 separating the apices of the inner and surrounding those of the outer pillars, and 

 constituting part of the membrana reticularis (figs. 18 and 13, tb), extend between 

 the future heads of the inner and outer rods of Corti and sever not only their 

 adjoining surfaces (figs. 14, 15 and 16, tb), but also the contiguous surfaces of the 

 heads of the two rows (fig. 9, tb). Under the influence of this modified cellular 

 cement (the terminal bars) there appear to develop firm, compact bodies, originally 



