THE LATER DEVELOPMENT OF THE FROG 151 



auditory epithelium. The wall of the auditory sac is only one 

 cell in thickness, except in its medio-ventral region where the 

 ganglionic portion of the placode is located. A small finger- 

 like outgrowth of the sac extends a short distance dorsally, from 

 its medio-dorsal region: this is the rudiment of the cndolyni- 

 yhatic duct. From this condition the auditory sac changes very 

 little until after the opening of the mouth (10-12 mm.) when it 

 continues its development. 



The next differentiations of the auditory sac result from the 

 formation of various ridges and septa extending into its cavity 

 (Fig. 51, C, D, E}. The first of these appears obliquely along 

 the outer and posterior walls of the sac, and finally divides the 

 otocyst into two regions, an inner and upper part, known as 

 the utricle, and a lower and outer part, the saccule; these two 

 divisions remain connected by a small perforation in the septum. 

 The endolymphatic duct is connected with a dorsal extension 

 of the saccule, while from the utricle the three semicircular 

 canals grow out. These canals are formed first by the growth 

 of couples of ridges into the cavity of the utricle, approximately 

 in the relative positions of the future canals; the couples meet 

 and fuse save at their ends, so that the cavities enclosed by them 

 open directly into the utricle. Each rudiment then begins to 

 enlarge and pushes above the surface of the utricle, first as a 

 ridge, which becomes plate-like and then tunneled between 

 the canal and the wall of the utricle. The posterior canal is 

 formed somewhat later than the anterior and the horizontal 

 canals. The ampullae are added to the canals by additional con- 

 strictions of the wall of the utricle. 



Shortly after the appearance of the semicircular canals, the 

 saccule commences to differentiate. First there appears a 

 postero-ventral outpocketing, the rudiment of the lagena or 

 cochlea, which forms a simple sac of considerable size. Then 

 posteriorly to this appears a second ventral extension, the 

 basilar chamber (pars basilaris). 



Upon the division of the auditory sac into utricle and saccule, 

 the endolymphatic duct remained in connection with the latter. 

 The duct slowly elongates dorsally, along the surface of the 



