150 OUTLINES OF CHORDATE DEVELOPMENT 



aa 



pa 



VIII 



FIG. 51. The development of the auditory organ in the frog and toad. A, 

 B, F, after Krause; C, D, E, after Villy. A. Section through the auditory 

 vesicle of an embryo just beginning to elongate. B. Section through the audi- 

 tory vesicle that has very nearly separated from the' superficial ectoderm. C. 

 Transverse section, somewhat oblique, through the auditory organs of a 12 mm. 

 R. temporaria. D. Slightly more advanced stage than C. E. Section through 

 the auditory organs of a 25 mm. R. temporaria. F. Membranous labyrinth of 

 the toad (Bufo vulgaris). a, Auditory sac; aa, anterior ampulla; ac, anterior 

 vertical semicircular canal; 6, pars basilaris; d, dorsal outgrowth of primitive 

 auditory vesicle (rudiment of endolymphatic duct); e, endolymphatic duct; 

 g, ganglion of auditory (VIII) nerve; he, horizontal semicircular canal; I, lagena 

 or cochlea; pa, posterior ampulla; pc, posterior vertical semicircular canal: 

 s, saccule; ss, sinus superior; u, utricle; VIII, auditory nerve. 



