142 THE FROG. 



The inner wall of the auditory vesicle, facing the brain, is 

 from the first composed of cells which are more columnar in 

 shape than those of the rest of the vesicle (Fig. 75, ev) ; and 

 it is with these elongated cells that the auditory nerve is 

 connected. As the vesicle grows, and as the septa form, by 

 which it is divided up into its various portions, the patch of 

 epithelium with which the nerve is continuous also divides, 

 giving rise to all the sensory patches present in the adult ear. 

 Of these there are eight : — one in each of the three ampullao of 

 the semicircular canals, three in the cochlea, one in the wall of 

 the sacculus, and one in that of the utriculus. 



The dorsally directed diverticulum, to which the pyriform 

 shape of the vesicle in its early stages is due, persists in the 

 adult, and undergoes a rather remarkable development. 



On the formation of the septum, dividing the vesicle into 

 sacculus and utriculus, the diverticulum remains in connection 

 with the inner side of the sacculus. It elongates considerably, 

 growing upwards close alongside the brain as the recessus 

 vestibuli (Fig. 75, er). In tadpoles of about 20 mm. length, 

 the distal blind end of the recessus vestibuli dilates to form a 

 thin-walled vesicle, lying on the roof of the fourth ventricle; 

 while the rest of its length forms a narrow tubular duct with 

 rather thick walls, which connects the dilated end with the 

 sacculus. 



At the time of the metamorphosis the distal thin-walled 

 dilatation, or saccus endolymphaticus, has increased greatly; 

 it lies within the skull, between this and the brain, as a large 

 sac with thin but very vascular walls, covering the roof of the 

 hind-brain for a considerable length, and extending downwards 

 along the sides of the brain and beneath its floor as well. The 

 sacs of the two sides meet, both above and below the brain, and 

 apparently open into each other; in their cavities abundant 

 calcareous concretions are found. 



The stalk, or ductus endolymphaticus, persists as a narrow 

 tube, which passes through a hole in the skull wall, and connects 

 the saccus endolymphaticus with the sacculus of the internal 

 ear. These relations of the saccus and ductus endolymphaticus 

 are retained in the adult frog. 



In the mesoblast surrounding the internal ear the perilymph 

 spaces are formed ; and beyond these the cartilaginous and 



