No. I.] THE VERTEBRATE EAR. 175 



ectoderm and its sinking to the bottom of the saucer-like depres- 

 sion. The whole process of the formation of this cup is an exact 

 repetition of the formation of a canal organ, and its canal as seen 

 in Amia, or of the formation of an ampulla and its canal in the 

 Salmon. As the saucer sinks below the surface, the opening on 

 the surface grows smaller, the bottom of the saucer increases in 

 size, and the resulting structure is an auditory vesicle distinctly 

 flask-shaped. The neck of the flask grows longer, and finally 

 appears bent backwards and inwards by the increase in size and 

 length of the head region which carries the auditory capsule 

 forward and outwards. During this lengthening of the neck of 

 the flask, which is the endolymphatic duct of the adult ear, the 

 body of the flask is much changed in shape. First of all it 

 becomes compressed laterally, then it is drawn out in an antero- 

 posterior direction, and concomitant with this elongation the an- 

 terior part points outward and the posterior part inward towards 

 the median line. There may now be seen several changes in 

 the shape of the vesicle, which, while not conspicuous, are very 

 important, since they usher in a succession of transformations 

 which ultimately produce the three semicircular canals and the 

 rudiments of the cochlea. These changes are visible on the 

 outer and upper faces of the vesicle as slight ridge-like eleva- 

 tions of the surface, and on the posterior ventral end of the 

 vesicle as a knob-like prominence. Of the former there are two, 

 of the latter a single one. The former structures seen from 

 the inner face of the vesicle are merely depressed grooves in 

 the wall of the vesicle, and the latter a sunken but broadly 

 open pit. These grooves and pit grow deeper, the lips of the 

 grooves come closer together, and finally in contact along the 

 middle part of their course the edges of the pit are drawn closer 

 together, narrowing its mouth. The lips of the grooves finally 

 come in contact, fuse, and the grooves are converted into canals, 

 opening into the auditory vesicle at each end. The auditory 

 vesicle has become indistinctly divided into two parts by the 

 formation of a constriction which arises somewhat obliquely off 

 from both of the vesicles, and marks an anterior and a posterior 

 chamber of the vesicle. From the lower part or sacculus the 

 outgrowing pit or rudimentary cochlea has now become a deep 

 pocket or bhnd canal, one side of which grows faster than the 

 other, causing it to curve on its own axis. The canals which 



