298 O. VAN DER STRICHT 



zontal fibrils, which, close to the apex of the inner hair cells, 

 are continuous with the more vertical fibrils of the subjacent 

 head lamella. Such structures depend upon the extension of 

 the head-plate in a definite direction, i.e., from a fixed point 

 corresponding to the seat of the central corpuscle close to the 

 outer hair cells, towards the inner acoustic elements. 



Some sections tangential to the surface of the organ of Corti 

 give pictures which prove that the head-plate is formed of two 

 superposed planes, one deeper and fibrillated (figs. 8 and 9, 

 ipP'-), the other more superficial, destitute of fibrils, and com- 

 posed only of a clear homogeneous cytoplasm (ipl^), imperfectly 

 enclosed by a part of the above-mentioned firm head-collar 

 (fig. 8, ipl). 



Structure of the heads of the inner and outer pillars in adult animals 



The ultimate structural changes undergone by the heads of 

 the pillars consist mainly in a broadening and extension of their 

 collars. This band not only becomes thicker, but also extends 

 over the head, to form the roof of the outer and inner pillar 

 head (fig. 10, ohd, ihd). This roof appears to correspond to the 

 'plaque cultioulaire' of N. Van der Stricht. Due to such trans- 

 formation, the collar becomes converted into a head cap, a firm 

 exoplasmic zone which circumscribes a clear granular endo- 

 plasmic zone, except at the medial side of the inner, and at the 

 lateral side of the outer pillar. In other words, the remainder of 

 the previous cytoplasm having now become much clearer, 

 occupies a cephalic notch (fig. 18,- ohd, ihd) which extends from 

 the head roof towards the pillar body; the bottom and the lips 

 of the groove are represented by the broadened head collar. 

 The clear endoplasm filling up the notch is traversed by the 

 fibrillar bundles of the heads, which in selected preparations 

 stain deeply with iron hematoxylin. 



The head of the inner pillar in the white rat (fig. 17, ihd) and 

 in the conmion rat (fig. 18, ihd) is thus fonned of a superficial 

 thinner, and a deeper, enlarged segment, both composed of a 

 medial groove containing a lamella of fibrils and a lateral layer 

 of firm, dark, homogeneous cytoplasm- — the walls of the notch. 



;J 



