PROPORTIONS OF THE TECTORIAL MEMBRANE 53 



along the outer edge of the membrane and is even attached to 

 this edge by the sparse fibrils formed by its cells before their 

 differentiation is completed (fig. 7). In the adult the hair cells 

 in the apical turns come to stand under the middle of the wide 

 outspanning zone (fig. 2). 



Pressure displacement of tissue in the direction of the least 

 resistance is common in organogenesis and, in the adult, such 

 displacement is a common observation of the surgeon. Compari- 

 son of the stages of the pig in the different turns of the cochlea 

 shows that the feet of the pillars separate during the growth 

 of the organ. The separation is greatest toward the apical end. 

 Hensen ('71 and '73, cited from Held) found by measurement 

 in the ox that the feet of both pillars are moved axisward. In 

 the apical turn he found that the foot of the outer pillar shifted 

 about 37 At and that of the inner pillar about 95 fi, thus not only 

 indicating actual movement but also accounting for the separa- 

 tion of the feet during the enlargement of the organ. I do not 

 know between what stages of ox fetuses the measurements were 

 made. If two elements of the spiral organ move axisward at 

 all, it is possible that all the elements may move. If the ele- 

 ments move at all, it is possible that they may move sufficiently 

 to account for the change in the relative position of the organ. 

 During the rapid increase in the width of the greater epithelial 

 ridge, the differentiating spiral organ is moved outward. 



The cells of the greater ridge, when it is widest (in pigs from 

 14 to 16 cm.), are about twenty-five times greater in number 

 than the cells, derived from the ridge, which later line the internal 

 spiral sulcus. The cells lining the sulcus in the adult are broader 

 than the cells of the greater ridge, but they average certainly 

 not more than three times as broad. As shown above, at from 

 13 to 16 cm., the outer third of the greater ridge is thickened 

 into a rounded elevation, its cells and nuclei being displaced 

 apexward and its outermost cells being forced in the outward 

 direction (figs. 7 and 8), indicating considerable growth pressure. 

 In the later recession and disintegration of the cells of the greater 

 ridge, as the production of the tectorial membrane is com- 



