AND THE CRISTA SPIRALIS OF THE COCHLEA. SI 



In addition to these two figures, 23 and 35, of adult stages, I give another, 

 similar, figure 36 and 36', from a young dog, and one. 37, from a pig embryo 150 mm., 

 with similar structures, longitudinal (cy), obliquely tangential (cy"), and transverse 

 sections (cy') of prisms. They are larger and the intraprismatic and interpris- 

 matic fluid is more abundant. As a matter of fact, in some instances in these 

 figures, instead of prisms with fluid content, there seem to be present transverse 

 sections of solid filaments, and in transverse sections (cy'} darker granules like 

 sections of fibers within the walls of the cylinders. Further investigation is wanted 

 to decide if these elements correspond either to real structures or to products of 

 the shrinkage induced by fixing agents, and if some delicate fibrils or bridges may 

 persist within the pale intraprismatic or interprismatic fluid. 



From this description I must conclude that the adult membrana tectoria is 

 formed throughout its extent by prisms or cylinders consisting of an outer dense 

 membrane or wall and of an axial clear fluid. They are separated by a more fluid 

 intermediary substance, which may be lacking in some places upon the surface of 

 the middle segment, but chiefly of the other segments, while a wall common to two 

 neighboring prisms separates their pale content. This superficial layer is the densest 

 and formed earliest in the course of development. Fixing agents, by reason of 

 the consequent shrinkage, seem to have a great influence on these structures and 

 induce many alterations. 



The results of these investigations permit me to emphasize the analogy of 

 structure and origin of the membrana tectoria and enamel of teeth. Both organs 

 are cuticular formations derived from a columnar epithelium, and their immediate 

 anatomical substratum is represented in the case of the former by the superficial 

 mosaic of the cells, and in the latter instance by a basal mosaic, the cytoplasmic 

 polygons of which are also separated by terminal bars according to the investi- 

 gations of Cohn (1897). Both are formed by a system of prisms, each produced 

 from one cell. The axial, more fluid part of the prisms of the tectorial membrane 

 is derived from the cytoplasm and the outer denser wall from the terminal bars, 

 which at the same time give rise to an interprismatic fluid. The solid calcified 

 prisms of the enamel are produced by the cytoplasmic base of the ameloblasts, 

 while the part taken by the terminal bars in the origin of the calcified cement has 

 not thus far been demonstrated. It should be pointed out that one epithelial cell 

 of the crista spiralis, and perhaps of the organ of Corti, may produce or be in con- 

 nection with many cylinders. 



After development the enamel becomes completely detached from its embryo- 

 logical substratum. The membrana tectoria, on the other hand, while becoming 

 free in greater part, keeps its primitive connections with the crista spiralis, the least 

 active part of its anatomical substratum. 



These results concerning the genesis of the tectorial membrane are of general 

 biological interest, for they prove the importance of the intercellular substance, 

 the terminal bars, in the origin of special constituents of organs considered as of 

 cuticular nature, now established for the membrana reticulares of sensorial organs. 

 In this respect the intimate structure, fibrillar or reticular, of the prisms or of some 

 segments of the membrana tectoria must be regarded as of secondary interest. 



