Development of the Third Day 185 



as a pair of small pits pushed in from the 

 surface ectoblast,in the region of the hind-brain, 

 (Fig. 47, V A). These auditory pits rapidly 

 deepen and, by the end of the third day, they 

 close together and become entirely separate 

 from the surface ectoblast which fuses over 

 them so that they are not visible from the 

 surface (Fig. 62, Ot.d.). These closed cavities, 

 lined with epithelium, are the auditory or otic 

 vesicles and from them, by a process of twisting 

 and unequal growth, the membranous labyrinths 

 of the ears are formed. The ends of the audi- 

 tory nerves very soon come in contact with 

 the epithelial linings of the auditory vesicles, 

 and by the early part of the third day they 

 have fused with them. The places where 

 this fusion occurs will be subsequently de- 

 veloped into the special patches of sensory 

 epithelium found in the adult ear. 



The development of the tympanic mem- 

 brane, tympanic cavity, and Eustachian tube 

 will be described in another place, when the 

 fate of the gill arches and clefts is described. 



It will be remembered that the auditory 

 vesicle in the frog was formed from the 

 inner or nervous layer of epithelium, and 

 that the vesicle was never open to the 



