78 THE GENESIS AND STRUCTURE OF THE MEMBRANA TECTORIA 



Figures 19 and 34, from a pig embryo of 127 mm., illustrate better the appear- 

 ance of these formations. The two tangential sections are a little oblique and 

 involve four successive planes. The first belongs to the nuclear and cytoplasmic 

 layer of the zona papillaris (zp); the second and more superficial shows the 

 superficial mosaic (mcr) of the zona dentata; the third exhibits wider areas (mi'} 

 enlarged by the fixing agents. 8ome of these areas contain only homogeneous 

 clear fluid, obviously a product of the superficial cytoplasm; they are separated 

 by thick, coarse lines as dark in the figures as the much thinner lines belonging to 

 the subjacent mosaic (mcr), the real terminal bars, although in my preparations 

 they are but faintly stained by Congo red. This dense part of the tectorial mem- 

 brane more recently formed (mt') is derived from the bars and is in direct conti- 

 nuity with a delicate secondary network of the same nature (and therefore of the 

 same origin) covering some of these clear areas. Finally, a fourth quite super- 

 ficial plane (mt) represents the older part of the membrana tectoria and its con- 

 stituent structures, but is more compact and reminds one of the structures in the 

 membrane produced by the greater ridge. 



The vertical sections, figures 14, 20, and 21, illustrate also the fact that the 

 tectoria membrane (mt) on the surface of the crista spiralis is formed by a dense 

 part, the uninterrupted walls between small cavities or chambers filled by a clear 

 fluid. On the right of figure 20 the dense part is marked by long lines which are 

 in continuity with similar constituents or cylinders belonging to the free membrane 

 overlying the sulcus spiralis. 



The features just described force me to conclude that at this stage of its develop- 

 ment the membrana tectoria of the crista spiralis is formed by a system of chambers 

 or cylinders with a fluid content derived from the superficial cytoplasm of the 

 epithelial cells, and by denser walls produced at least in part by the terminal bars. 

 Indeed, as already stated above, during the development of the membrana olfac- 

 toria limit ans, of the membrana reticularis of the crista and macula acusticse, and 

 of the membrane covering the apices of the inner supporting cells in the organ of 

 Corti, the substance of the bars extends over the neighboring fields and may form 

 a kind of delicate network which gives rise to secondary structures of the same 

 nature and of the same chemical composition as those of the bars. 



In the case of the developing tectorial membrane the primitive bars generate 

 on their surface a coarse primary network of a different chemical composition, the 

 large meshes of which contain a secondary, more delicate reticulum of the same 

 nature and of the same chemical composition. This secondary network is derived 

 at least in part from the primary, perhaps in part also from similar cytoplasmic 

 structures. To what extent the apices of the epithelial cells take part in the genesis 

 of this delicate reticulum I am no more able to determine than Alice Thing has 

 been (1917) in regard to a similar secondary network generating the fundamental 

 substance of the zona pellucida in the turtle. 



Many series of preparations of adult and embryonic stages showing features 

 similar to those of figures 14, 19, 34, 20, and 21, prove that a great number of cylin- 

 ders of the free membrana tectoria reach and are connected with one cytoplasmic 



