No. I.] DEVELOPMENT OF THE EAR OF AMBLYSTOMA. 27 



greatly accelerated growth, the relation of parts is changed, and 

 the division appears to be of a dorsal utriculus and a ventral 

 sacculus. At the time the canals are first differentiated the 

 posterior one opens by its proximal (ampullar) end into the sac- 

 culus. Later, as the utriculo-saccular constriction reaches the 

 mesal side of the vesicle, the canal becomes connected with the 

 utriculus alone. The horizontal canal opens originally by its 

 distal or amaP end into the sacculus, but is transferred by the 

 same utriculo-saccular constriction to the utriculus, and passes 

 into the latter just ventral to the distal end of the posterior 

 canal. This distal end of the horizontal canal is always much 

 larger than the corresponding ends of the other canals. The 

 ampullae appear simultaneous with or but little later than the 

 differentiation of the canals, as distinct enlargements of their 

 respective canals. The ampulla of the horizontal canal is from 

 the beginning the largest of the three, while the anterior am- 

 pulla is the smallest. The cristse acusticae bear the same rela- 

 tions in size to each other as their respective ampullae. The 

 ampullae of the anterior and horizontal canals open directly 

 into the utriculus, being marked off from the same merely by 

 constrictions. But the posterior ampullae is separated from 

 the utriculus by a canal of some length, the sinus posterior utri- 

 culi of Retzius. As this connecting tube is developed with the 

 posterior canal and as such is the part of the latter differentiated 

 from the sacculus, it seems more fitting to regard it as a con- 

 necting tube (Verbindungsrohre) of the posterior ampulla with 

 the utriculus, the corresponding part in the horizontal and the 

 anterior ampullae being greatly shortened. The utriculus after 

 its very early stages is essentially cylindrical. In early develop- 

 ment it is relatively much larger than in the adult. Fig. 20 

 represents the relations of utriculus and sacculus retained in 

 the adult. 



In connection with the mesal wall of the sacculus are devel- 

 oped three diverticula, with each of which is connected a nerve- 

 end organ. These diverticula are known as the lagena, the pars 

 neglecta, and the pars basilaris. 



Lagena. — The lagena is the first of these to appear. It is 

 indicated even before the Anlagen of the semicircular canals 

 make their appearance, by a ventro-mesal protuberance or 



^ See Avers, op. cit. 



