LECTURE XX. 



THE ORIGIN OF THE EAK. 



Pro/. Oliver P. Jenk'utx. 



The auditory organ. What we learn through it. 



Outline of its structure. External ear. Middle ear. 

 These two are accessory organs. Their functions. 

 Internal ear. Position and relations of the auditory 

 cells and the auditory nerves fibers. These are the 

 essential parts. 



Outline of the development of the human ear. Its 

 development compared with that of the eye. Origin 

 of the essential elements out of the upper germinal 

 layer. 



The ears of vertebrates ; that of birds ; of reptiles ; 

 of batrachians ; of different kinds of fishes. Each of 

 these compared with the human ear. 



In the development of the human ear successive 

 stages are reached comparable to a series of ears of 

 lower vertebrates. 



Auditory organs of vertebrates. Their diversity in 

 form and location. Their community of origin from 

 the outer layer of cells (epidermal). 



Organs of sense of smell, of taste, and of touch ex- 

 amined briefly as to their structure and development. 



Each shows that the essential elements have arisen 

 from the outer layers of cells. 



