OUR BEST FEATURES 



in the shape of two small papillae which grow out 

 from the side of the sac below the semicircular 

 canals. In the crocodiles and alligators one of 

 these papillae is prolonged into a curved tube 

 (Fig. 104D) and in the mammals (Fig. 104E, F) 

 the tube is wound into a spiral, the cochlea. Thus 

 while the semicircular canals which are concerned 

 with balance show only minor changes as we pass 

 through the long series from shark to man, the 

 organ of hearing in air has its beginnings in the 

 Amphibia and culminates in the typical mammals, 

 from which it is transmitted intact to the apes 

 and man. 



The chamber of the middle ear (Fig. 106) in 

 the frog (which represents a comparatively little- 

 modified survivor of the earliest amphibians) is 

 derived in the embryo from an out-pocketing from 

 the throat, corresponding to the first or hyoid 

 gill pouch of fishes. This chamber is therefore 

 lined with the entoderm, or primary inner cell 

 layer. The Eustachian tube of the frog is the 

 short passage connecting the cavity of the middle 

 ear with the cavity of the throat. By this arrange- 

 ment the outward pressure of the air inside the 



mouth and throat neutralizes the inward pressure 



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