DEVELOPMENT OE THE EAR IN THE COMMON EROG. 163 



2. The Hyomandibular Cleft, Eustachian Tube, and Tympanic 



Cavity. 



3. The Annular Cartilage, the Stapes, and the Columella. 



a. The Semicircular Canals. — These canals .are essentially parts of 

 the auditory vesicle which have become separated from the main 

 part by septa which are incomplete, inasmuch as each canal opens 

 into the vesicle at each end of the septum. Each septum originally 

 consists of two entirely distinct folds which grow together from 

 opposite walls of the vesicle, and by their coalescence cut off the 

 canals except at their ends. The anterior vertical and the horizontal 

 canals develope simultaneously in the frog, whilst the posterior 

 vertical is distinctly later in formation. 



The first indication of the canals is afforded at about 11 mm., by 

 folds of the walls of the vesicles projecting internally. These folds 

 are at first purely epiblastic ; but very soon irregular mesoblastic 

 cells migrate into the space included in each double fold. The 

 ridges so formed are placed one at the anterior end, in the upper 

 and inner part of the vesicle, and one opposite this and projecting 

 towards it from the upper and outer part, and running back nearly 

 to the hinder part of the ear. In its hinder half this latter projects 

 ventrally instead of horizontally ; in fact, it is almost completely 

 divided into two parts, the anterior corresponding to the first ridge 

 mentioned, and with it going to form the septum of the anterior 

 vertical canal, whilst the posterior portion lies opposite to a third 

 fold projecting upwards from the ventral part of the vesicle, these 

 two constituting the septum of the horizontal canal. 



The two folds of each pair grow towards each other till they meet 

 and coalesce for a considerable distance. At the stage now reached 

 the anterior vertical and horizontal canals are already established. 

 Figs. 7 and 8 show two stages in the formation of the canals as here 

 described. In Fig. 7 the septa have not yet been completed, whilst 

 in Fig. 8 the constituent folds have met and blended. It should be 

 noted, however, that these two sections do not exactly correspond, 

 for they cut the head at different levels. The septa formed in this 

 way rapidly thicken and elongate, thus lengthening the canals, this 

 lengthening keeping pace with the general growth of the vesicle 

 during this period. 



Whilst these changes have been going on the posterior vertical 



