58 HUMAN EMBRYOLOGY AND MORPHOLOGY 



otocyst, subdivides into the saccule and utricle (Fig. 44). The 

 division occurs at the entrance of the endolymphatic canal, which 

 thus comes to open into both saccule and utricle. The endo- 

 lymphatic canal, which is simply the stalk of the otocyst, is 



jjjj-ductus endolymphaticus 

 -semcircular canal 



primitiue utricle 



cochlear canal 



Fig. 45.— The Otocyst in an Embryo of five weeks ; it shows a demarcation into the 

 various parts of the Membranous Labyrinth. 



enclosed in the petro-mastoid, its extremity appearing at the 

 hiatus vestibuli, where it ends beneath the dura mater in a 

 dilatation. It appears to be a vestige of the primitive mouth of 

 the otocyst, which opens on the surface of the head in many 

 fishes. Lastly, the scala media or canal of the cochlea (Fig. 44) 

 grows out from the saccule. There is merely a rudiment of the 

 cochlea in fishes and amphibians. In reptiles, birds, and mono- 

 tremes it is a straight canal — the Lagena. Only in mammals 

 is it arranged spirally. In it the organ of Corti is developed. 



The Petro-mastoid. Origin.— The mesoblast surrounding the 

 membranous labyrinth becomes cartilaginous at the end of the 

 3rd month of foetal life, forming the periotic capsule (Figs. 35 

 and 39). The tissue which immediately surrounds the mem- 

 branous labyrinth does not undergo chondrification, but becomes 

 converted into an open meshwork of cells, the intercellular spaces 

 containing perilymph. The lymphatic space thus formed within 

 the petro-mastoid, containing the saccule and utricle, is the 

 vestibule. The scalae tympani and vestibuli of the cochlea are 

 of similar origin. An oval space on the outer wall of the 

 vestibule is not chondrified ; it contains the footplate of the stapes- 

 and forms the foramen ovale. The foramen rotundum also 

 remains unchondrified and separates the tympanum from the: 

 scala tympani (Fig. 43). 



