PROPORTIONS OF THE TECTORIAL MEMBRANE 37 



surface of the sections of the further developed membrane, or 

 it perhaps represents a part of Held's Decknetz. 



In the outer side of the thick epithelium, there usually appears 

 at this stage a lighter area (L, fig. 5) resulting from the nuclei 

 here being placed farther away from the apical ends of their 

 cells. Comparison with later stages (figs. 6 and 7) suggests 

 that this area represents the outer margin of the greater epithelial 

 ridge. 



In fetuses of about 8 cm., the increase in number of the cells 

 in the epithelium of the basal side of the cochlear duct has ren- 

 dered the thicker epithelium there still thicker and wider, and 

 has also resulted in a second but lesser thickening along the 

 outer edge of the first. At this stage the cells of both the greater 

 and the lesser epithelial thickenings, or ridges, are engaged in 

 the production of the finely fibrillar, tangled structure charac- 

 teristic of the first and seemingly imperfectly formed part of 

 the tectorial membrane. The cells of the lesser ridge, which 

 later become differentiated into the elements of the spiral or- 

 gan, seem to retain but a very short time this tendency to pro- 

 duce fibrils similar to that of their adjoining neighbors of the 

 greater ridge, along with whom they have developed. While 

 producing fibrils, they are never as actively productive as even 

 the cells of the extreme axial edge of the greater ridge, and 

 their product is more sparse and less mature than even the first 

 product of the greater ridge. They represent the edge of the 

 productive grading into the non-productive cells, and, as they 

 differentiate into the cell elements of the spiral organ, they cease 

 production altogether. 



In figure 6, from a pig fetus of 9 cm., the first evidence in my 

 series of the differentiation of the elements of the spiral organ 

 is shown. The first to be distinguished is the series of inner 

 hair cells {I AC). While in some sections of this stage and earlier, 

 there may appear shrinkage cracks between the cells of either 

 of the ridges, the beginning of the spiral tunnel has not appeared 

 at this stage, though Prentiss represents the tunnel in his figure 

 of the second turn of a pig of 5.5 cm. Figure 6, as does each of 

 the figures following, represents a section through the 3rd half 



