Chap. VII.] THE GENESIS OF TISSUES AND ORGANS. 135 



cross section, and ends blindly near the top. The scala tympani 

 and scala vestibuli also get smaller, and eventually communicate 

 one with the other at the top of the spire at a. Thus it would 

 be possible for a minute organism to swim out of the perilymph 

 surrounding the vestibule into the scala vestibuli, and so up to 

 the top of the spire, and then down again by the scala tympani. 

 At the foot of that canal his way would be blocked by a mem- 

 brane filling an aperture in the bone of the osseous labyrinth, 

 and called the fenestra rotunda. If he were to swim back into 

 the perilymph round the vestibule he would there find a similar 

 membrane filling the aperture of the fenestra ovalis. And this 

 membrane is connected by means of the chain of auditory ossicles 

 (as shown in Fig. 43, C.) with the tympanic membrane. The 

 vibrations of the tympanic membrane are therefore passed on 

 by the ossicl6s (see p. 147) to the membrane of the fenestra 

 ovalis, which sets all the perilymph in vibration, and with it the 

 membrane of the fenestra rotunda. The partition between s. t. 

 and s. v. in 43, B., contains a bony shelf, the lamina spiralis, but 

 those between these canals and the cochlear canal are mem- 

 branous, while that between s. t. and c. c., called the basilar 

 membrane, contains radial fibres over-arched by bowshaped fibres 

 of Corti, and arranged somewhat as in the key-board of a piano. 

 It has been suggested that they may have the function of vibrat- 

 ing to sounds of a certain definite pitch ; but they cannot be 

 described here. 



In the fowl there is no external ear ; the tympanic membrane 

 is placed in connection with that of the fenestra ovalis by a 

 columella ; there is a fenestra rotunda ; the sacculus and utricu- 

 lus are less completely constricted off; the cochlear canal 

 is not spiral, as in the rabbit, but is a curved process with a 

 dilated extremity. The osseous cochlea is, as compared with 

 the rabbit, rudimentary. 



The auditory organ, like the olfactory organ and the eye, 

 originates as a depression of the surface of the head. This 

 auditory pit becomes converted into a closed epiblastic vesicle by 

 the closure of its edges and the overgrowth of the superficial 

 epiblast. From this otic vesicle and its mesoblastic investment 



