DEVELOPMENT OF THE ORGAN OF CORTI 297 



tion as illustrated in figures 7 and 8 {ihd) indicates an oblique or 

 more or less longitudinal section of the pillar, and such prepa- 

 rations are liable to misinterpretation. 



The most remarkable changes occur at the level of the free 

 apices of the inner pillars, the summit of the pillar scaffolding. 

 The gradual development of the head of the outer pillar, situated 

 just beneath this summit, produces a radial extension of the 

 latter, and the transformation of a very small square field (figs. 

 1, 2, and 3, aip) into a long narrow fibrillated membrane or head- 

 plate. This gradual extension is clearly shown in figures 4 (aip), 

 7 (ipl) and 8 (ipl\ ipl'^), whereas no enlargement in a spiral 

 direction is noticeable. On measuring the radial diameters of the 

 fibrillated head-plates in figures 3, 4, 7, and 8, and comparing them 

 with the radial diameters of those portions of the membrana 

 reticularis included between the plates and the outer border of 

 the apices of the third row of acoustic elements, it is found that 

 the former are respectively represented by about 1/11, 1/2.75, 

 1/2, and 1/1.64 of the latter. This statement gives a rather 

 accurate picture of the rapid enlargement of the head of the 

 outer pillar and the subsequent extension of the superficial inner 

 pillar plate; that is, of the portion of the membrana reticularis 

 formed by the latter during the development of the tunnel space. 



From this description it is also evident, according to N. Van 

 der Stricht (p. 610), that the extension of the apex of the inner 

 pillar is due solely to a mechanical factor, a compression by the 

 underlying enlarging head of the outer pillar. This view has 

 been corroborated by Held ('09). He does not mention the 

 description given by N. Van der Stricht but states (p. 212): 

 ''Je mehr der Kopf des Aussenpfeilers sich bildet und in seiner 

 Masse wachst, um so diinner wird iiber ihm die Kopfplatte des 

 Innenpfeilers." In its extension the head-plate undergoes im- 

 portant structural changes. Originally foniied of a clear cyto- 

 plasmic field (figs. 1 and 3, aip) containing a diplosome or two 

 central corpuscles, the elongating plate becomes subdivided into 

 two zones, a lateral, small, clear zone, enclosing the diplosome 

 (fig. 7), and a medial, more extensive, fibrillated one. This 

 continues to lengthen and is composed of several parallel hori- 



