206 



MARINE BIOLOGICAL LABORATORY. 



then drawn out in an antero-posterior direction. During 

 this process the anterior end comes to point outward, 

 while the posterior end is directed inwards toward the 

 median line. There may now be seen several changes 

 in the shape of the vesicle which, while not conspicu- 

 ous, are very important, since they usher in a succession 

 of transformations which ultimately produce the three 



aw* 



Fig. 12. — The internal ear of a some- 

 what older Pig embryo, ^;^ mm. long. 

 Figure after Krause. 



Fig. II. — The ear of a very 

 young Rabbit embryo, 1 1 mm. 

 long. Figure after Krause. 



a Anterior canal. 



atn Ampulla. 



d Ductus endolymphaticus. 



/ Lagena. 



5 Sacculus. 



u Utriculus. 



semicircular canals and the rudiment of the cochlea. 

 These changes are visible on the outer and upper faces 

 of the vesicle as slight ridge-like elevations of the sur- 

 face, and on the posterior ventral end of the vesicle as a 

 knob-like prominence (Fig. lo, c, c/i, and a.v). Of the 

 former there are two, of the latter a single one. The 



