THE ORGANS OF THE OUTER GERM-LAYER. 



493 



rl 



am (>:b) 



^ 



"in" 



vV 



hb 



dc 



It is difficult to follow the formation of the otoliths within the 

 otocyst. In one case, which FOL was able to follow, they were 

 developed by an epithelial cell in the wall of the vesicle. The cell 

 secretes small calcareous concretions in its protoplasm, becomes 

 enlarged in consequence, and protrudes as an elevation into the 

 endolymph. When it has become more heavily loaded with calcic 

 salts, it is connected with the wall by means of a stalk only, and 

 finally it becomes entirely detached from the wall and falls into the 

 cavity of the vesicle, in 

 which it is kept float- 

 ing and rotating by the 

 ciliate cells. 



In Vertebrates the 

 otocyst, which, as we 

 have seen, agrees in its 

 first fundament with 

 the organ of hearing 

 in Invertebrates, is con- 

 verted into a very com- 

 plicated structure, the 

 membranous labyrinth, 

 -the evolution of which 

 in Mammals I shall de- 

 scribe in some detail. 

 It undergoes metamor- 

 phoses, in which the 

 formation of folds and 

 constrictions plays the 

 principal part (fig. 276). 

 The auditorv sac de- 



/ 



tached from the epi- 

 dermis, and lying at the 

 side of the after-brain, soon exhibits a small, dorsally directed pro- 

 jection, the recessus labyrinthi or ductus endoly niphaticus (fig. 275 rl). 

 Probably we have to do in this with the remnant of the original 

 stalk by means of which the auditory vesicle was connected with the 

 epidermis. According to some investigators, on the contrary, the 

 .stalk disappears entirely and this evagination is a new structure. 

 The first assumption is favored especially by the previously mentioned 

 condition in the Selachians the presence of a long tube, which 

 maintains a permanent connection between labyrinth and epidermis. 



Fig. 276. Membranous labyrinth of the left side of a 

 [human] embryo, after a wax model by KRAUSE. 



//, Recessus labyrinth! ; dc, ductus cochlearis ; hb, pocket 

 from which the horizontal semicircular canal is formed ; 

 a in', enlargement of the pocket which becomes the 

 ampulla of the horizontal canal ; am (rb), vb', * com- 

 mon pocket from which the two vertical semicircular 

 canals are developed ; am (vb), enlargement of the 

 common pocket from which the ampulla of the an- 

 terior vertical canal arises. An opening (w) has been 

 formed in the pocket, through which one sees the 

 recessus labyrinthi. * Region of the pocket which 

 becomes the common arm of the two vertical canals 

 (sinus superior) ; rb', part of the common pocket which 

 furnishes the posterior vertical canal. 



