GROWTH OF THE INNER EAR OF ALBINO RAT 29 



in each instance the breadth of the outer zone, and the sum of 

 the two inner parts that of the inner zone, while the sum of all 

 four parts gives the total radial breadth. For the purpose of the 

 exact measurement of the growth of the membrane, I have, 

 as noted above, projected the sections at 100 diameters and made 

 the determinations on the outlines thus obtained. 



In table 4 (charts 2 and 3) are given the values for the total 

 average breadth, as well as for that of each zone, and also the 

 thickness of the membrane, from 1 to 546 days of age. At the 

 bottom of each column are given the ratios of the breadth at 

 1 to 546, 12 to 546, and 20 to 546 days. While the ratio between 

 1 and 546 days is 1.7, those from 12 to 546 days and 20 to 546 

 days diminish to about 1:1.0, that is the membrane at twelve 

 days has attained about its full breadth, and there is only a 

 very gradual increase in its breadth with advancing age. After 

 twelve days similar ratios are found for the separate zones as 

 well. 



From 1 to 546 days the ratios for the two zones differ con- 

 siderably; that for the second zone is 1:1.2 and that for the 

 first is 1 :3.6. This is due to the fact that in the cochlea at birth 

 the development of the labium vestibulare is incomplete, even 

 in the basal turn, while at the apex we can very often hardly 

 see the invasion of the mesenchymal tissue in the inner part of 

 the greater epithelial ridge. 



At every stage the outer zone is broader than the inner; the 

 ratio between them at birth is 1:3.8. This diminishes to 1:1.25 

 at twelve days, after which age it remains practically constant. 

 Owing to the form of the membrana tectoria and to its great 

 sensitiveness to the method of preparation, it is difficult to 

 obtain good values for its thickness. 



Generally speaking, the membrane is thickest: about midway 

 between the outer edge of the labium vestibulare and the inner 

 boundary of the inner hair cell, and it was here the measurements 

 given in table 4 were made. As shown in this table, the thickness 

 increases rather rapidly from birth to twenty days, but after 

 that period remains approximately constant. 



