DEVELOPMENT OF THE ORGAN OF CORTI 299 



This lateral layer enlarged rapidly toward the lower pole of the 

 head and tapers downward to blend with a small unifonn pro- 

 toplasmic zone of the pillar body (fig. 17, ipb). In the cochlea 

 of the adult bat, similar structures are seen (fig. 13'^, ihd), but 

 near the surface of the head (fig. 13'""*^, ihd) only a very thin 

 fibrillated lamella is recognizable. However, in vertical spiral 

 sections showing the longitudinal fibrils throughout the length 

 of the pillars, a part of the head cap is visible (fig. 16, ihd). 



The notch of the outer pillar head enlarges from the roof 

 towards the pillar body and presents a true asyimnetrical 

 position (figs. 17 and 18. ohd), and so is the structure of the head 

 cap itself. On cross-sections the lips of the groove differ in 

 thickness, the apical (i.e., that turned toward the apex of the 

 cochlea) being obviously thinner than the basal (i.e., that 

 turned toward the base of the cochlea). The clear cytoplasm 

 of the notch is traversed by the nearly horizontal fibrils of the 

 phalanx process (fig. 17, ohd), the rootlets of which merge 

 obliquely into the apical lip. The other bundle of fibrils, run- 

 ning vertically from the pillar body toward the surface of the 

 head, also shows an asymmetrical position. On passing into 

 the head this bundle proves to be bipartite, being formed of a 

 smaller and a larger fasciculus (fig. 13'^-^, ohd). Within the 

 lower and wider portion of the notch (fig. 13'^, ohd) the sub- 

 division into two unequal fasciculi is more evident, and the two 

 bands are more closely connected respectively with the apical 

 and basaJ lip of the groove. At the level of the head roof the 

 horizontal fibrillated bundle courses through the cleft between 

 the two vertical fasciculi, each of which merges into its neigh- 

 boring lip (figs. 13''' and 17, ohd). Most of these asymmetrical 

 structures may be recognized during the development of the 

 head (figs. 4 and 7, op). In vertical spiral sections of the adult 

 organ, the asynometry is very conspicuous. In figure 14 the 

 apical surface (the surface turned toward the apex of the cochlea) 

 of the head is clearly indicated by the course of the apical fila- 

 ment of the cells of Deiters {ap, d^) in the direction of the apex of 

 the cochlea. In such sections (figs. 14 and 15) can be seen the 

 clear, eccentric oval notch, which contains a cross-section of the 



