76 THE GENESIS AND STRUCTURE OF THE MEMBRANA TECTORIA 



apices and the pale fluid must be considered as derived from this cytoplasmic 

 substratum. The longitudinal, thick, irregular lines overlie the apices of the sup- 

 porting fields better visible in figure 5, op, d', d", d'", where the terminal bars between 

 two neighboring hair-cells of the same row seem to form a single thick line; while 

 the thinner transverse lines overlie those bars, also irregular, visible between the 

 different rows. Hence the longitudinal and transverse lines of the first super- 

 ficial plane (s') should be considered as derived from the subjacent terminal bars 

 that is to say, from the real membrana reticularis of the organ of Corti. 



I believe, consequently, that I am justified in drawing the conclusion that the 

 membrana tectoria is formed at the surface of the organ of Corti by the same 

 process as that observed on the surface of the greater ridge. The differences 

 in appearance of the figures are induced by corresponding differences in the 

 anatomical substratum. 



The second more superficial plane is also very instructive. It consists of two 

 fields (s") identical in size with those of the preceding (s'). Their longitudinal lines 

 (0 are undoubtedly double and the two neighboring halves are connected by short 

 bridges. The transverse lines can not be recognized and a delicate pale network 

 is visible within the clear areas. The structures of this plane resemble those of 

 the third superficial plane (s'"), where the longitudinal lines have also disappeared, 

 and the constituents are the same as those of the neighboring membrana tectoria 

 belonging to the greater ridge (mtg). 



What is the significance of the pale network appearing within the clear spaces 

 of the second plane (s")? Two different explanations may be given. The network 

 represents the most superficial portion of the membrane formed in the early stages 

 of its development, before the appearance of the hair-cells, when the subjacent 

 mosaic is formed by undifferentiated polygons (figs. 1, 2, ml). Figure 33, from a 

 pig embryo of 150 mm., shows the membrana tectoria recently produced (mf) on 

 the surface of the second and third rows of Deiters cells (//", d'") and the third 

 row of outer hair-cells (oh'"). One sees clearly (mf) a series of pale round fields 

 surrounded by very thick bands which, with similar lines around smaller areas, 

 reproduce more or less the subjacent membrana reticularis of the lateral part of the 

 organ of Corti. This figure proves that in the course of development the struc- 

 tures of the tectorial membrane change here greatly, as do those of the subjacent 

 membrana reticularis, and that the differences between its first (mi) and its later- 

 formed layers (mf) become gradually more pronounced. I am thus induced to 

 accept this first explanation. 



A second explanation would bring me to consider this delicate reticulum as 

 derived from the bars in such way that the dense part of the membrana tectoria 

 directly formed by them and covering them may grow and extend over the apices 

 of neighboring cells and there give rise to structures like those on the crista spiralis. 

 I will refer to this question later. 



Held (1909) describes a system of thick fibers derived from the apices of the 

 sustentacular cells of the organ of Corti and chiefly from the first and second rows 

 of Deiters cells; these filaments he terms "Haftfasern der Lamina reticularis 



