238 HOWARD AVERS. [Vol. IV. 



opening. The sense organs, in the meantime, have become 

 differentiated out of the indifferent epithelium, and present the 

 appearance of a rounded elevation covered with hair or rod- 

 bearing sensory, columnar, epithelial cells. 



The key to the solution of the whole problem lies in explain- 

 ing the development of the internal ear on the basis of the 

 development of the canal organs of the surface of the body. 



When we examine the development of the canals and cochlea 

 from the primary auditory vesicle, in the light of Allis' investi- 

 gations, we find that the whole process consists in the further 

 development within the head of one of the depressed areas and 

 its sense organs, that are found in numbers on the body (head 

 region) of all embryo vertebrates, and just as the primary sense 

 organs of the surface of the body may (and usually do) produce 

 several or many organs, so does the organ inclosed within the 

 vesicular involution of the ear continue to multiply, until it has 

 produced the constituent parts of the adult ear of a given verte- 

 brate form. 



Some of the deductions from these premises are as fol- 

 lows : — 



a. Each natural group of vertebrates at the present day has 

 a type of internal ear closely adhered to by all its members. 



The so-called single semicircular canal of Myxine being, in 

 reality, a double structure possessing two ampullae and ampullar 

 sense organs, the homology of parts between Myxine and 

 Petromyzoii is essentially complete. 



b. The semicircular canals, among the Elsamobranchs, many 

 times show a condition of canal development, indicative of the 

 fusion of the primary pores at the two ends of the same canal, 

 with an incipient separation of the primary pore thus formed 

 from the rest of the ear. 



c. In any study of the functions of the internal ear, we can- 

 not lose sight of primitive function of the sense organ giving 

 rise to sensory apparatus of the ear. For the functions have 

 not suffered greater alterations than the structures, and there is 

 no difficulty whatever in making out the structural relations of 

 the primary and finished organs. 



d. If the function of equilibration, as supposed, belongs in large 

 part, or entirely, to the semicircular canals and their ampullar 

 organs on account of their spatial relations, we cannot overlook 



