The Nature of the Tectorial Membrane 141 



the curved course of the fibers apparent in sections of it. All sections 

 of foetuses show that the stages of development advance from the basal 

 coil toward the apical, and so in the increase in thickness and width 

 of the greater epithelial thickening, beginning at the base and advanc- 

 ing toward the apex of the cochlea, must lie the explanation of the fact 

 that the outer direction of the fibers is inclined from the radial toward 

 apex. Measurements from my preparations show that between 10 cm. 

 and 15 cm., the width of the greater epithelial thickening increases 

 from 124 microns to 174 microns. This increase consists of a very 

 evident increase in the number of the epithelial cells concerned. 



At about 14 cm., the lesser epithelial thickening, the future organ of 

 Corti, has considerably advanced in form and appears as if it acts as 

 a block to the further increase in width of the greater thickening, for, 

 at that stage, the outer edge of the latter becomes bluntly rounded and 

 bows upward (Fig. 10) with the result that in this edge, adjacent to 

 the lesser thickening, there is formed the thickest part of the greater 

 thickening. The outer edge of the developing tectorial membrane 

 necessarily bends downward around this bluntly rounded edge of its 

 parent epithelium. Up to this stage, the membrane never overlaps the 

 lesser thickening and in confirmation of the statement of Eickenbacher, 

 it must be said that at no stage is there good reason to assume that the 

 cells giving rise to the organ of Corti ever have any thing to do with 

 its development. 



The order of the development of the membrane seems to be, first, 

 the fibers and then the matrix, both being subadded gradually at the 

 under surface. In all stages, from the very first, there always exists 

 upon the immediate surface of the epithelium a thin, clear, parallel 

 layer in which the matrix is either in a soluble stage of its develop- 

 ment or wholly absent, and across which course the ends of the fibers 

 from the epithelial cells into the body of the membrane above. In 

 sections the thus unembedded ends of the fibers appear exceedingly 

 tender, are often broken free and usually appear in agglutinated bundles 

 (Fig. 10). 



The stage of retrogression of the greater epithelial thickening, and 

 the resultant displacement of the parts, begins with a cessation of 

 growth of the epithelial cells, followed by their exhaustion and finally 

 by a liquefaction of their remains, till, in the adult cochlea, the cells 

 of the greater thickening are represented by the very low, indifferent 

 epithelium which lines the spiral sulcus from the inner supporting 



