No. I.] THE VERTEBRATE EAR. 189 



the pocket for the horizontal canal are present. Below the 

 pocket for the horizontal canal is seen the larger and deeper 

 evagination for the cochlea. When the embryo has reached a 

 length of 8 mm., the cochlea has so far developed as to stand 

 out distinctly from the sacculus, and is at the same time some- 

 what bent. 



At both ends of the common pocket for the verticals, and at 

 one end of the horizontal depressions, are slight enlargements, 

 the future ampullae. From now on the parts rapidly acquire 

 individuality. The anterior canal is completed first, then the 

 posterior, then the horizontal. The whole vesicle undergoes a 

 marked change during this period, for owing to the growth of 

 the ductus endolymphaticus from the utricular part and of the 

 cochlear tube from the saccular division, the labyrinth has now 

 the appearance of being extended in a dorso-ventral direction. 

 The cochlear canal has now one-half turn, and a long groove 

 has appeared on the inside of its wall (fold on the outside), from 

 which the nerve-end organ, Corti's organ, arises. 



In embryos of 30 mm. length the canals are all well formed, 

 and the ampullae prominent. The two verticals, which from 

 their mode of development have up to this time occupied the 

 same plane, now begin to diverge, and the planes of the canals 

 now meet in the middle of the common arm which unites them. 

 Their angle is 150°. This method of origin is most interesting 

 and important in its bearing on the spatial relations of the adult 

 ear canals. The cochlea has now nearly one spiral turn. 



Krause failed to observe the separate formation and was in 

 error with reference to the origin of the two canals — anterior 

 and posterior verticals, from the fact that at the time when the 

 evagination to form the canals has become well marked the two 

 grooves have run together and hence appear as a single fold. 

 The truth is that there are two ampullar depressions which 

 give rise to outgrowing canals. These canals having their 

 separate origins are to be regarded as distinct, though they fuse 

 at so early a period. ^ Krause looks upon the resorption of the 

 intervening tissue as a necessary step in the formation of the 

 semicircular canals. Notwithstanding Krause's assertion that 

 the ampullae form at the same time as the canals, I am certain 

 that the ampullar depressions make their appearance before 



1 See Recapitulation D. 



