J 32 AYERS. [Vol. VI. 



ist anatomisch gleichwertig der Basalmembran, einem Aus- 

 scheidungs-produkt epithelialer Zellen." 



In these words von Noorden distinctly shows that he has 

 utterly failed to grasp the significance of the formation of the 

 primary and secondary folds, and the histological differentiation 

 which he looks upon as the cause of the canal formation is cer- 

 tainly only a secondary matter, a concomitant of the erection of 

 the folds. 



The further development of the labyrinth is brought about 

 by the continuous growth of the arches formed in the manner 

 described above into broad sheets, which convert the archways 

 into canals. It is important to note that von Noorden found 

 in a majority of cases that the two processes uniting to form 

 the posterior canal usually came together last, though he thinks 

 he has observed this pair to unite first of the three, and he 

 concludes that there is no regularity in the process, especially 

 as he had oftentimes observed the right and left capsules to be 

 in widely separated stages of development, the right labyrinth 

 being much more advanced than the left. The portion of the 

 canal first formed is its ampulla, since the folds first meet over 

 the sense organ or crista. We shall see later on that this fact 

 has an important bearing on the solution of the difficult prob- 

 lem of the origin of the part in question in the higher forms, — 

 the mammals especially. 



The size of the processes forming the rudimentary canals 

 varies considerably in the different species, but there are no 

 fundamental variations in the relations of the parts. 



The Development of the Ear in the Amphibia. 



In the Frog the early history of the auditory vesicle is in all 

 essentials, save one, a repetition of the development as given 

 for the Shark. The exceptional condition is that the auditory 

 vesicle develops entirely from the nervous layer of the ecto- 

 derm, and is always closed over by the epidermal layer. The 

 endolymphatic duct consequently never opens on the surface of 

 the head. This condition is retained by all the Sauropsida, and is 

 certainly a secondary modification of the developmental process. 



According to Villy (1890, 281), who has recently worked out 

 the early stages of the development of the ear in the Frog very 



