PROPORTIONS OF THE TECTORIAL MEMBRANE 43 



gressively wider and thicker toward the apical end of the cochlea 

 and remains active longer at the apical end, thus producing the 

 progressively broader and thicker apical end of the membrane. 

 Figure 9, drawn from a section of the 3rd half turn of the cochlea 

 of a pig of 22 cm., suggests the persistent activity of the outer- 

 most part of the greater ridge. In it, the final retrogression of 

 the producing cells (GR) is represented. This retrogression 

 seems to occur after the cells of the axial two-thirds of the ridge 

 have completed their retrogression and decrease in number to 

 form the cells lining the spiral sulcus. The indentation in the 

 basal surface of the membrane over the disintegrating cells is 

 thought to be artefact, for tracing backward through the stages 

 suggests that ver}^ little if any of this region of the membrane is 

 contributed by the outer part of the greater ridge. The proc- 

 ess by which the spiral organ assumes its position under the 

 outspanning zone of the membrane is about completed and, in 

 the process, the cells which produced the outer regions of the 

 membrane have been carried axisward from their product. Be- 

 tween the membrane and the organ and disintegrating cells, 

 there is, in some of the sections of this stage, a suggestion of deli- 

 cate fibrils, some of which seem to be drawn axisward. So, if 

 that portion of the membrane which now lies over the disinte- 

 grating cells is contributed to by the cells at all, it is most prob- 

 ably only a few delicate fibrils drawn axisward, parallel against 

 and contributing to the peripheral condensation of the basal 

 surface. 



Attention is called to the fact that the spiral organ has in- 

 creased in size between the stage of 19 cm. (fig. 8) and 22 cm. 

 (fig. 9). Though its growth is not quite completed, it has ap- 

 proached the thickness possessed by the adult in this turn of the 

 cochlea. The growth increase in the thickness of the organ is in- 

 dicated in table 4 in which averages of the thickness are given 

 in micra. The measurements were taken from the surface of 

 the organ, through the middle of the outer hair cells, to the basi- 

 lar membrane. 



Cochleae from one litter of pigs averaging about 21 cm. showed 

 the differentiation of the spiral organ completed in the 3rd half 



