50 IRVING HARDESTY 



spanning zone of the membrane is found during its production, 

 in all trustworthy preparations of the pig, overlying the cells 

 of the greater ridge only and associated with them only. The idea 

 is very simple but I am convinced the actual anatomy in devel- 

 opment does not support it. From my study of the speci- 

 mens I, as yet, find no evidence that the few fibrils seen 

 coming from the lesser ridge and continuous w^ith the outer 

 edge of the membrane proper can contribute appreciably, if 

 at all, to the bulk of the membrane. I think they disinte- 

 grate for the most part at least, leaving the outer edge of the func- 

 tioning membrane clean and bluntly rounded, its fibrils curving 

 from the apical surface around the edge and axiswaVd in the 

 basal surface. Both Rickenbacher's and Held's figures, from 

 other animals, show what I consider the outer edge of the later 

 tectorial membrane extending only to the inner hair cells. 



Comparison of the different stages suggests that each of the 

 other three ideas is tenable in part, two of them especially; that 

 the relative position of the organ to the basal surface of the 

 membrane does become shifted and that three factors may enter: 



(1) The growth increase in thickness of the vestibular lip 

 of the spiral Umbus can have nothing to do with inducing the 

 membrane to span the spiral organ. • Averages obtained from 

 measurements in several cochleae of each of six stages from 13 

 cm. to the adult show that in all the turns of the cochlea, the 

 lip acquires its adult thickness in the fetus of 14 to 15 cm. At 

 this stage the membrane proper is still being produced in all 

 the turns of the cochlea and does not anywhere project over the 

 spiral organ. The thickness of the lip in the different half 

 turns in the pig of 14 cm. and the older stages, including the 

 adult, were found to be strikingly similar. Further, the vestib- 

 ular lip attains less thickness throughout in the apical turn 

 than in the basal, while the membrane projects over and beyond 

 the organ more in the apical turns. So it is hardly probable 

 that the membrane is tipped outward by increase in thickness 

 of the vestibular lip of the spiral limbus. The width of the vestib- 

 ular lip, while difficult to measure definitely, likewise seems to 

 increase very little between 14 cm. and the adult. The small 



