58 THE GENESIS AND STRUCTURE OF THE MEMBRANA TECTORIA 



bats and dogs show that it is formed by the terminal bars, and Alice Thing (1917) 

 considers that the very thick zona pellucida of the ovum of the turtle is produced 

 by the terminal bars of the surrounding epithelium. They extend over the free 

 surfaces of these cells, where appears a delicate network of the same nature as 

 that of the bars. This network, together with the bars, gives rise to the cuticular 

 fundamental substance of the zona pellucida. 



Enamel and the membrana Corti or membrana tectoria are included in a third 

 group. In both cases the adult organ becomes completely detached from its 

 generating substratum, the first from the bases of the ameloblasts, each of which 

 produces a kind of cuticular prism (the enamel prism) . These elements are sepa- 

 rated by the calcified cement substance which is considered to be a kind of inter- 

 cellular product, although its origin has not been clearly described. The second, 

 the membrana tectoria, becomes detached from the surface of the greater and the 

 lesser epithelial ridges in the cochlear duct and remains fixed to only the least active 

 portion of its generating substratum, the crista spiralis. Held (1909), discussing 

 the nature of the membrana Corti, thinks that the membrane should not be con- 

 sidered cuticular, not because its layer first formed is not homogeneous, but because 

 its constituent elements, its fibrils, as they become more and more elongated, pro- 

 ceed from the cytoplasm as different plasmic products and not as cell prolongations. 

 A cuticle, he states, is not represented by flagella, by cilia of a ciliated epithelium, 

 or by sensorial hairs. In addition a cuticle always remains attached to the surface 

 of the cell. Hence Held regards the membrana tectoria as a specific product of 

 the free surface of the greater and lesser epithelial ridges, the sensorial cells of which 

 do not take part in its development. Therefore the fibrils of the membrana Corti 

 can not be termed cuticular. Held seems to forget the recognized fact that enamel 

 prisms are real cuticular elements, although they become completely detached from 

 their anatomical substratum. 



The object of my research is the study of the development and structure of 

 the membrana tectoria. Although this problem has received the attention of many 

 investigators, it seems to me that the results obtained have been rather contradic- 

 tory and give for the most part no satisfactory interpretation because of differences 

 between the morphological substratum and the real structure of the membrane 

 derived from it. Recent investigators have more or less neglected the structure 

 of the crista spiralis. I intend to devote to it special attention. 



METHODS. 



I have investigated the following material: Pig embryos of 60.0, 93.5, 95.0, 

 127.0, 137.0, 150.0, and 190.0 mm.; a new-born dog; young kittens; the following 

 adult animals: bat, dog, rat, and mouse. 



The isolated cochlea was fixed by one of the following agents: Trichloracetic 

 acid, 5 per cent in water; this decalcifies bone very well after one, two, or three 

 days, according to the size of the cochlea; Benin's fluid; Zenker's fluid. 



After fixation by the first agent, the pieces were transmitted directly to abso- 

 lute alcohol, to which some drops of iodine had been added. After remaining one 



