64 THE GENESIS AND STRUCTURE OF THE MEMBRANA TECTORIA 



cellular cement (Kittsubstanz) in black, Lavdowsky expressed the opinion that the 

 membrana reticularis is formed by this metamorphosed substance. 



Figure 8, a section tangential to the surface of the crista acustica in a new- 

 born dog, shows a system of thick terminal bars (tb) between the smaller polygonal 

 supporting fields (suf), each of which presents a central corpuscle and the larger 

 more circular sensorial fields (sf) , within which a dark central plate and an eccentric 

 central corpuscle (cp) are visible. 



Figure 9 represents a similar appearance of the crista acustica in an adult bat 

 (Vespertilio fuscus). Here the bars (tb) are very much enlarged and extend over 

 the greater part of the clear sustentacular fields, leaving uncovered only their 

 central area (suf) . The more or less circular openings (suf) of the membrana retic- 

 ularis become smaller in figure 10, the crista acustica of an adult white rat, and 

 are narrowest in figure 11, the macula acustica of an adult mouse. The much 

 larger sensorial fields (sf) of these last three figures show the central dark cuticular 

 plate from which proceed the hairs traversing these large openings of the membrana 

 reticularis. The power of enlargement and extension of the originally thin termi- 

 nal bars is fully demonstrated by these four figures, as also is the real origin of the 

 fenestrated membrane derived from them. 



As regards the origin of the membrana reticularis of the organ of Corti, figures 

 4, 5, and 7 are noteworthy. During the earliest stages in the process of develop- 

 ment of the membrana tectoria the bars separating sensorial and supporting fields 

 are rather thin, although much thicker than those visible in my preparations at the 

 surface of the cells of Hensen and of Claudius, but they gradually enlarge, chiefly 

 after the membrana tectoria is formed. In the adult organ of Corti one sees (fig. 7) 

 well how the bars have become thicker everywhere and are enlarged most between 

 the first and second rows of outer hair-cells and in such a way that between these 

 two rows, and again between the second and third row, there is to be seen a system 

 of lines alternately thin and thick (tb"), but relatively thicker in the latter situation. 

 Finally, along the row of inner hair-cells between two neighboring sensorial fields 

 (fig. 7', ih), there is a small, dark veil hiding the apices of the inner supporting 

 cells. This originates as an extension of the terminal bars, and I was able to see 

 similar figures and superficial veils in the organ of Corti of adult rats and dogs. 

 The development of this veil is another striking proof of capacity for extension 

 over neighboring cells possessed by the terminal bars. It may be recalled here 

 that the existence of a small plate or a prolongation of the head of the inner pillar 

 has been mentioned previously by Retzius (1884) and Held (1902). According 

 to Retzius, who did not recognize the inner supporting fields, it extends between 

 the apices of two inner hair-cells. Held, who terms this prolongation a rostrum 

 (Schnabel) or a bill, describes it as spreading over a small outer zone of the inner 

 supporting cells. I presume that this rostrum is a part of the superficial veil which 

 I have described as derived from the terminal bars, but which these two investi- 

 gators did not recognize. 



During the development of the membrana tectoria the terminal bars, as already 

 stated, possess the power to grow and thicken upon the surface of the greater 



