164 FRANCIS VILLY. 



canal has been forming. The first signs of its presence are apparent 

 some time after the ridges forming the septa of the other canals have 

 appeared, but before the coalescence of these ridges. It is formed 

 exactly as the other canals are ; the outgrowths developing, one 

 on the septum between the sacculus and utriculus hereafter to be 

 described, and situated on its outer edge directed upwards and 

 inwards ; the other is on the inner wall of the vesicle, more than 

 half way up it and opposite to the first. This is shown in Fig. 8. 

 These folds coalesce after the time of cutting off of the two other 

 canals. In the adult the posterior vertical canal curves over the 

 horizontal canal and opens into the utriculus below it. This is 

 brought about by the formation of the hinder ampulla, which by its 

 growth causes an apparent inversion of these parts. The part of the 

 utriculus originally above the horizontal canal developes into the 

 ampulla, and in so doing bulges out and encircles the canal. 



The two vertical canals are at first entirely distinct, but as they 

 lengthen they approach each other and meet. In this way a common 

 length is formed, short at first, but gradually elongating, and through 

 it both canals open into the utriculus. 



With this stage the adult relations of the canals are attained, 

 further change being limited to their change in size, and the growth 

 of mesoblast between them and the rest of the auditory vesicle, 

 whereby they are removed farther and farther outwards and attain 

 their characteristic curves. The mesoblast concerned consists at 

 first of irregular scattered cells, but as the cartilaginous auditory 

 capsule developes the cartilage extends between the canals, and in 

 tadpoles of about 20 mm. it entirely surrounds them. 



With regard to the relative times of formation of the canals, the 

 anterior vertical and horizontal are formed simultaneously, but the 

 posterior vertical is only just commenced when the other two are 

 nearly cut off, and there are stages in which the posterior vertical 

 canal is not fully separated by the blending of the folds constituting 

 its septum, whilst the other two have been formed some time. It is 

 noteworthy that of the sensory epithelia of the ampullae, that of the 

 posterior is the first to be specialised, though the corresponding 

 canal is the last to develope. This will be more fully described and 

 an explanation offered in dealing with the differentiation of the 

 sensory epithelium of the ear. 



