162 FRANCIS VILLY. 



can be recognised from the moment the wall of the sac is completed, 

 and it results from the mode of involution, but it is not a stalk 

 connecting the vesicle with the exterior at any stage. The typical 

 Vertebrate ear retains its opening to the exterior for some time by 

 means of such a recessus labyrinthi, and the duct may even be 

 present in the adult, putting the endolymph in communication with 

 the surrounding medium (Elasmobranchs) ; but when the ear-fold 

 concerns the inner epiblast alone no such communication takes place 

 at all. In the case of the frog we should expect the recessus to be 

 the last part of the vesicle to retain its connection with the external 

 skin, but this is not so. This irregularity may perhaps be explained 

 by the double-layered condition of the epiblast, which condition is 

 most probably a secondarily acquired one. 



The epithelium forming the vesicle is more or less columnar at all 

 points. The older part, i.e., that forming the wall next the brain, 

 the floor and the recessus, consists of cells differing from the 

 remainder in being more elongated. This is most marked at the 

 lower inner angle, where the cells are deeply pigmented. This tract 

 of elongated cells extends upwards and outwards at the anterior end, 

 especially in the later stages of this period. 



The auditory nerve runs from the inner wall, with the greater 

 part of which it is connected, and reaches the brain just behind the 

 recessus. The part applied to the vesicle is expanded into a ganglion 

 of some size, whilst the nerve connecting it with the brain consists 

 at this stage of a fairly thick strand of cells. The whole is pig- 

 mented. During this period, or at least nearly the whole of it, 

 until the tadpole is about 11 mm. in length, the auditory nerve is 

 absolutely continuous with the facial nerve. After this stage the 

 ear becomes too complicated to describe clearly as a whole ; I propose, 

 therefore, to take the various parts one by one and follow them up 

 to the adult condition in this order : — 



1. The Differentiation of the parts of the Internal Ear. 



a. The Semicircular Canals. 



i. The Sacculus. 



c. The Cochlea. 



d. The Epithelium of the Auditory Vestibule. 



e. The Eecessus Labyrinthi. 

 /. The Perilymphatic Spaces. 



