140 . THE FROG. 



nerves and the special patches of epiblast with which they 

 become fused. 



Certain of the accessory organs of hearing, especially the 

 Eustachian tube and the tympanic cavity, may conveniently be 

 described here, although they are essentially independent of 

 the auditory apparatus, and only become secondarily connected 

 with this. 



The early development of the ear. About the time of closure 

 of the neural groove, the auditory epithelium can be recognised 

 as a pair of thickened circular patches of the deeper layer of 

 epiblast, one at each side of the hind-brain, with which patches 

 the auditory nerves are already continuous. 



Soon after closure of the neural tube, in embryos of about 

 3 mm. length (Fig. 60), each of these patches becomes depressed, 

 forming a shallow pit, semicircular in transverse section, and 

 covered at its mouth by the outer layer of epiblast, which is 

 continued over it without interruption. The pit deepens, and 

 the mouth gradually closes by ingrowth of its lips. Shortly 

 before the hatching of the tadpole the closure is completed, and 

 the auditory vesicle separates from the surface epiblast. 



At the time of its separation the vesicle is a closed sac, some- 

 what pyriform in shape ; its lower or ventral portion being 

 spherical, and lying opposite the notochord, and its dorsal wall 

 being prolonged upwards into a short blind diverticulum lying 

 at the side of the hind-brain. The wall of the vesicle consists 

 of a single layer of cubical or columnar cells ; those of the 

 inner wall, with which the auditory nerve is continuous, being 

 rather more elongated and more deeply pigmented than the rest. 



The internal ear or labyrinth. After closure of its mouth the 

 vesicle increases considerably in size, and becomes further sepa- 

 rated from the surface by ingrowth of mesoblast between its outer 

 wall and the external epiblast. Up to the time of the formation 

 of the mouth it undergoes no further change of importance, 

 remaining as a spherical sac with a blind dorsal diverticulum. 



Shortly after the opening of the mouth, i.e. in tadpoles of 

 from 10 to 12 mm. in length, the various parts of the internal 

 ear become gradually differentiated, the chief process by which 

 the changes are brought about being the formation of septa, by 

 folding of the wall of the vesicle, which project inwards into 

 the cavity and partially subdivide it. Mesoblast soon grows in 



