DEVELOPMENT OF THE EAR IN THE COMMON FROG. 167 



to all the sensory patches present in the adult ear, although at the 

 time under consideration it is continuous. 



The sensory patches of the ear are eight in number. Three of 

 these are situated in the ampulla), three in the cochlea, and of the 

 remaining two one is on the floor of the anterior part of the utri- 

 culus, and one on the inner and lower part of the sacculus. 



The first division of the columnar epithelium takes place at the 

 hinder end of the vesicle in tadpoles of about 11 mm. It occurs 

 on the formation of the oblique septum, which I have described as 

 marking the first trace of division of the vesicle into utriculus and 

 sacculus. The small sensory patch thus separated lies in the hinder 

 end of the utriculus, that is, where the posterior ampulla is formed 

 later, so that the ampulla as it developes includes this epithelial 

 patch. 



The fact that this epithelial tract is the first to develope might be 

 taken to show that though the posterior vertical canal actually forms 

 later than the others, nevertheless it is indicated earlier than these 

 are. This early specialisation may, however, possibly be explained 

 as a convenience due to the out-of-the-way position occupied by the 

 organ concerned and to the early formation of the septum between 

 the utriculus and sacculus, which in its growth plays an important 

 part in separating this patch from the rest. Besides, I do not think 

 that the time of specialisation of the sensory tracts of the ampullar 

 is a safe guide to the relative times of evolution of the canals, as 

 there is no rule of development common to all three. If the 

 ampullae were exactly homologous structures, and their development 

 had not been interfered with, they would form in the same way ; 

 but as they do not do so, it is safer to assume that the original 

 course of development has been modified than that the ampulla) 

 are not homologous. This would seem to show that the order of 

 the evolution of these epithelial patches has not been preserved. 



It is interesting to note that just as the anterior vertical and 

 horizontal canals form close together, and are even united from the 

 first, so the epithelium of their ampullae developes from a common 

 rudiment, which becomes separated from the main mass soon after 

 the posterior tract already mentioned. This rudiment, common to 

 tlie two ampullae, is situated on the outer wall at the anterior end of 

 the vc-icle, pear the point where the ampulla: of the two canals, with 



