THE EAR. 397 



At a stage a little later than that shown in Fig. 159, the 

 cochlear canal, which up to this point has been only slightly 

 curved, begins to form the spiral turns, so characteristic of the 

 adult (Fig. 157,c), the twisting being brought about by growth 

 in a spiral manner of the blind end of the canal. 



The cochlear canal becomes the scala media of the cochlea in 

 the adult. Immediately outside it the mesoblast becomes ex- 

 cavated to form a couple of tubular passages, the scala vestibuli 

 and scala tympani, which lie respectively above and below the 

 scala media or cochlear canal. The scala vestibuli and scala 

 tympani commence at the basal end of the cochlea, and gradually 

 extend along it towards its apex, following the turns of the 

 spiral ; and ultimately, on reaching the apex, they open into each 

 other, though not until a very late stage of development. From 

 the epithelium of the floor of the cochlear canal, or basilar mem- 

 brane, the organ of Corti is developed ; while the roof of the 

 cochlear canal, separating it from the scala vestibuli, is spoken 

 of in the adult as the membrane of Reissner. 



At the base of the cochlea, the scala vestibuli opens into the 

 peri-lymphatic space surrounding the central part of the 

 vestibule, while the scala tympani is closed at its base by the 

 membrane of the fenestra rotunda. 



Similar peri-lymphatic passages are formed, by excavation of 

 the mesoblast, around the semicircular canals. 



As in the frog, there is at first a single patch of the 

 epithelium of the auditory vesicle with which the auditory nerve 

 is continuous. This single, large patch becomes ultimately 

 broken up into several smaller ones, which by growth of the 

 intervening strips of epithelium are separated further and further 

 from one another until they reach their adult positions. 



The accessory auditory apparatus of the rabbit is, in a general 

 way, similar to that of the frog or the chick, but is more com- 

 plicated. 



The Eustachian tube (Fig. 157, ET) is formed from the 

 hyomandibular gill-pouch. This reaches very close to the surface 

 in the early stages of development, but does not open to the 

 exterior at any period. On the eleventh day (Fig. 158, HM) 

 the hyomandibular pouch reaches almost to the surface, the 

 hypoblast of the pouch meeting the epiblast at the bottom of 



