DEVELOPMENT OF THE EAR IN THE COMMON FROG. 165 



It is usually stated that the two vertical canals in the higher 

 Vertebrates are the first to develope, and that they correspond to the 

 two canals of the Cyclostomata. From this it is argued that the 

 Cyclostome ear is more primitive than others, a stage representing their 

 condition being passed through in the development of higher forms. 

 It will be seen from the above that this doctrine is very much shaken 

 by the results arrived at in this paper. 



The ampulla?, concerning the formation of which nothing has as 

 yet been said, are developed by the constricting off of parts of the 

 vesicle at the ends of the canals. The sensory epithelium of each 

 ampulla is present some time before the ampulla itself, and this 

 sensory epithelium is situated at the appropriate end of its canal. 

 As the ampulla? are formed, they therefore include their special 

 epithelial tracts from the very first. 



b. The Saccukts. — In tadpoles of about 11 mm. in length, just 

 before any signs of the semicircular canals appear, the internal ear is 

 complicated by the formation of a septum running obliquely across 

 its hinder part from the upper and outer to the inner and lower 

 angle. This septum seems to grow forwards from the posterior wall 

 of the vesicle, first appearing as a ridge and gradually broadening 

 till it divides the posterior part of the vesicle into a larger upper 

 and inner and a smaller lower and outer part (see Fig. 8). These 

 two divisions of the vesicle communicate with the rest of the cavity 

 in front, and therefore indirectly with each other. This septum is 

 the first sign of a division into utriculus and sacculus. The upper 

 portion of the vesicle becomes the hinder part of the utriculus in 

 the adult, that is chiefly the posterior ampulla. The septum under 

 consideration is at first confined to the posterior part of the vesicle, 

 and remains so for some time ; but later on it extends all round the 

 vesicle, and growing inwards completes the division into utriculus 

 and sacculus. This division is not quite complete, for the septum 

 remains perforated at its centre, thus leaving the utriculus and 

 sacculus in communication. The septum consists of a double epi- 

 blastic fold for the most part. It attains the proportions of tho 

 adult in tadpoles of about 20 mm. 



Although the sacculus is formed in part merely by its separation 

 from the rest of the vesicle by an incomplete septum, still it mani- 

 fests active growth, and the great downwardly projecting pouch of 



