GROWTH OF THE INNER EAR OF ALBINO RAT 155 



respect. The intensity of the stimulus is important in determining 

 the hearing reflex, as Small ('99) has stated. In my cases the 

 young rats responded very evidently to intense sounds, while 

 they reacted weakly or not at all to those which were faint. 

 Thus only the intense sounds were perceived by the rat of nine 

 days. 



Conclusions 



1. The hearing reflex was never obtained in rats less than 

 nine days of age. 



2. At nine days a single rat, one of five in a litter, responded to 

 a sharp sound like clapping the hands and to a whistle of high 

 pitch; the other four did not respond. At the tenth day some 

 of the four reacted, and at the twelfth day all could hear. 



3. The hearing reflex probably occurs early in young rats 

 that are vigorous and well nourished. 



4. To obtain the first hearing reflexes it is necessary to have 

 rather strong sounds of high pitch. 



5. A comparison of the histological structure of the cochlea 

 in rats of nine days, one of which could hear and the other could 

 not, shows clear differences in its development of the cochlea. 

 These consist not in the detachment of the tectorial membrane 

 from the papilla spiralis, but in the degree of differentiation of 

 the constituents of the papilla. The tectorial membrane is 

 connected in both cases at its outer end with the terminal frame 

 of the lamina reticularis by a thick thread. The papilla is more 

 differentiated in the hearing rat in several characters. The 

 tectorial membrane has reached with its outer end the outermost 

 row of the outer hair cells, but in the not-hearing rat it has not 

 yet reached the second row of the cells. 



6. The form of the papilla and its relation to the surrounding 

 structures, especially to the tectorial membrane, are in the hearing 

 rat at nine days very similar to those in the rat at twelve days 

 of age, though there are some differences between them in ab- 

 solute size. 



7. The freeing of the tectorial membrane from the papilla 

 spiralis is not necessary to the appearance of the hearing reflex, 



