166 FRANCIS VILLY. 



the sacculus is mainly formed as an outgrowth of the vesicle, This 

 is plainly seen just after the completion of the semicircular canals. 

 At this time a slight bulging of the floor of the vesicle may be 

 noticed near the anterior end of the horizontal canal. This forms 

 the pouch of the sacculus spoken of above. It enlarges rapidly, 

 and soon loses its distinct character, as its opening increases more 

 quickly than the remainder, so that it never becomes deeply con- 

 stricted off. 



c. The Cochlea. — Under this heading I include the three pouches 

 of the sacculus described by Hasse* and Retziusf as the lagena, the 

 pars neglecta, and the pars basilaris. There is very little of importance 

 to note concerning them except the relative times at which they 

 appear. The lagena is the hindermost and largest of the three. It 

 first appears as a small pouch at the lower and inner angle of the 

 vesicle, opening into it at about the level of the anterior aperture 

 of the posterior vertical canal (see Fig. 8). It early acquires 

 considerable size, and extends back some distance. As the peri- 

 lymphatic spaces form, as described below, the inner wall of this 

 pouch becomes thin and abuts against one of these canals, the part 

 of the pouch concerned presenting a flattened appearance. The 

 remaining parts of the cochlea develope simply as bulgings of the 

 walls of the sacculus. The first of these to form is the anterior and 

 upper one, which is situated close beneath the mouth of the recessus. 

 This is. called the pars neglecta. The pars basilaris is therefore the 

 last part to appear. 



d. The Epithelium of the Auditory Vestibule. — In this section I 

 propose to trace the formation of the patches of sensory epithelium 

 in the various parts of the ear. 



When the lining of the invaginated auditory vesicle begins to 

 differentiate, cells of two kinds may be distinguished, — columnar and 

 pavement. The columnar cells are disposed in a tract running the 

 length of the floor of the vesicle, and especially well marked at the 

 inner angle. This tract extends up the inner wall for a certain 

 distance, and in the fore part it runs up the outer wall. The 

 remainder of the vesicle is formed of a flattened epithelium. The 

 columnar portion of the epithelium, however, divides and gives rise 



* " Zeitschr. f. wiss. Zoologie," xviii., " Das Gehororgan der Frosche." 

 t " D. Gehororgan d. Wirbelthiere." 



