AND THE CRISTA SPIRALIS OF THE COCHLEA. 73 



seem to be homogenous and structureless; but on careful examination (ml', figs. 

 25', 26') the)- look double, as if split longitudinally into two parallel thin lines 

 severed by a clear space which is at times bridged across, the bridges being immersed 

 in a kind of intercellular substance. 



A tangential section through all the layers of the membrana tectoria (mtg) over 

 a large extent (fig. 28) proves that the same structures are visible everywhere, the 

 mazes of this kind of network becoming a little smaller toward the surface of the 

 membrane (on the right side of the figure) than in the vicinity of its inferior side, 

 next to the mosaic of the greater ridge. Nowhere can there be seen transverse 

 sections of filaments or of fibrils. Hence the reticulum in figure 28, and mi', figures 

 25 and 26, must be considered as the optical section of a system of walls, of mem- 

 branes surrounding cylindrical or prismatic tubules filled with a pale fluid. In other 

 words, the membrana tectoria is formed by a system of cylinders or prisms con- 

 sisting of a dense outer wall derived from the terminal bars and of a contained 

 portion, the more fluid part, derived from the cytoplasmic apices of the epithelial cells. 



This view is confirmed by vertical sections through the greater epithelial ridge, as 

 represented by figures 29, 30, and 31, taken from the cochlea of a pig embryo of 

 95 mm. These show the cylinders or prisms lengthwise as double lines (cy) and cut 

 across as circular or polygonal fields (cy'}. Their transverse sections in figures 30 

 and 31 are quite similar to those in figures 28, but the longitudinal sections (cy) 

 demonstrate better that between the cylinders exists an interprismatic clear sub- 

 stance with delicate structures, within which elements like bridges can be noticed. 

 Figure 29 shows most clearly that the young membrana tectoria consists of a basal 

 clear layer and one more superficial, darker and more compact, the walls of the 

 cylinders being denser near the surface and their diameters being a little smaller, 

 with an intermediary substance less abundant. Finally, the base of each prism is 

 obviously in continuity with one of the slightly prominent apices of the epithelial 

 cells; but these vertical sections can give no sure knowledge concerning the origin 

 of the constituents of the membrana tectoria. Therefore, tangential sections are 

 needed. 



We shall see that the interprismatic substance undoubtedly exists in the mem- 

 brane of adults; hence it is not an artificial product, the result of shrinkage. It is 

 derived from the terminal bars which may split longitudinally into two thinner 

 parts connected together by short bridges. The primitive bars close the subjacent 

 intercellular spaces and separate the intercellular cement from the interprismatic 

 substance, which must therefore be considered as derived from the bars themselves. 



Some authors, Coyne and Cannieu (1895), Hardesty (1908), Held (1909), and 

 Prentiss (1913), have drawn and described figures similar to those in my figures 

 25, 26, mt', and also regard them as representing the first stage of the developing 

 membrana tectoria. Hardesty and Held consider these figures as a network of 

 filaments derived from the superficial cytoplasm of the epithelial cells and forming 

 the fibers of the adult membrane. Hardesty (1915, p. 60) states: 



"In the production of the tectorial membrane each cell of the greater epithelial ridge 

 may contribute an average of 25 fibrils to the membrane. Each fibril seems to show a 



