OUR FACE FROM FISH TO MAN 



In the frog, a modern representative of the 

 Amphibia, there is no external ear tube, since the 

 tympanic membrane hes on the surface (Fig. 106). 

 In the reptiles a ridge or fold of skin may guard 

 the drum membrane and in the birds and typical 

 mammals the latter has sunk so far below the sur- 

 face that a deep tube is formed. 



That the mammalian outer ear tube corresponds 

 only at most in part with the spiracular pocket of 

 the shark is indicated by the fact that the outer 

 ear tube of mammals is formed below the Eustach- 

 ian tube (which represents the lower part of the 

 first internal gill pouch), while in fishes the spir- 

 acular pocket is formed from the upper part of 

 the spiracular cleft and lies above the first internal 

 gill pouch. 



In Echidna, one of the egg-laying mammals, 



G. Ruge found that the cartilage of the external 



ear was continuous with the hyoid, or second gill 



arch, and hence the inference was drawn that the 



external ear cartilage was derived from the hyoid 



arch. But Gaupp's figures of the embryo Echidna 



show the hyoid cartilage entirely distinct from the 



external ear. And the relations of the ear tube 



to the tympanic ring both in Echidna and in other 



210 



