AUDITORY ORGANS. 



431 



The scala vestibuli is formed before the scala tympani, and both 

 scalie begin to be developed at the basal end of the cochlea : the 

 cavity of each is continually being carried forwards towards the 

 apex of the cochlear canal by a progressive absorption of the meso- 

 blast. At first both scalae are somewhat narrow, but they soon 

 increase in size and distinctness. 



FIG. 304. SECTION THBOUGH THE INTERNAL EAR OF AN EMBRYONIC SHEEP 

 28 MM. IN LENGTH. (After Bottcher.) 



D.M. dura mater; R.V. recessus labyrinth!; H.V.B. posterior vertical semicircular 

 canal; U. utriculus; H.B. horizontal semicircular canal ; b. canalis reuniens ; a. 

 constriction by means of which the sacculus hemisphericus S. R. is formed; /. 

 narrowed opening between sacculus hernisphericiis and utriculus; C.C. cochlea; 

 C.C'. lumen of cochlea; K.K. cartilaginous capsule of cochlea; K.B. basilar plate; 

 Ch. notochord. 



The cochlear canal, which is often known as the scala media of the 

 cochlea, becomes compressed on the formation of the scala? so as to 



