l66 AYERS. [Vol. VI. 



D. Embryological. 



An account of the successive stages in the early development 

 of the internal ear in the following vertebrate types : — 



1. Pisces. Petromyzon, Galeus, Acipenser, Salmo. 



2. Amphibia. Rana. 



3. Reptilia. Lacerta, Tropidonotus. 



4. Aves. Gallus. 



5. Mammalia. Lepus, Ovis, Bos, Sus, Felis, Homo. 



TJie Ontogeny of the Ear in the Several Classes of Vertebrates. 



Valentine (1839, 279) first described the formation of the semi- 

 circular canals of the vertebrate ear. He states that they form 

 by the growth of blind pockets of the utriculus, in a semicircu- 

 lar manner, out from the parent cavity, through the tissue of the 

 head, until they come again in contact with the utricular wall, 

 when they break through the walls at the point of contact, and 

 by a fusion of the edges of the parts thus brought together estab- 

 lish communication by the curved channels between the two 

 ends of the utriculus. This outgrowth of the tubular processes 

 was followed by a wave of reduction of the bore of the canals, 

 which stopped short of the end of the canals and thus left 

 enlargements, the ampullae, which, of course, represented the 

 original size of the canals. According to Valentine, the poste- 

 rior canal was first to form, the anterior following it. Our 

 author did not observe the formation of the external canal. 



Rathke (1839, 223), from his studies on the Adder {Tropidono- 

 tus natrix), could not trace the development of the "semicircular 

 canals as Valentine had described it ; but found that each canal 

 was formed independently by the closing over of the edges of 

 a groove formed in the wall of the utriculus. The edges of the 

 grooves or folds united first near the middle of the length of 

 the groove and grew each way toward the ends of the utriculus. 

 It is at the point of contact that the separation of the canal 

 from the utriculus first takes place by the resorption of the 

 tissue composing the fused folds. The canals grow rapidly in 

 extent after separation begins. Three years later his results 

 were confirmed by Gunther (1842, 107) for mammals, and by 

 Bischofi (1842) for birds. 



