25 NORRIS. [Vol. VII. 



vesicle. The spaces shut off communicate with the main cav- 

 ity at both ends of the partitions, thus forming canals. The 

 first of these folds to appear are apparently those of the hori- 

 zontal canal. The dorsal fold of the septum of the horizontal 

 canal soon elongates and is continuous with the lateral fold of 

 the septum of the anterior canal. The posterior canal is formed 

 later than the others. Villy ^ states that in the frog the divis- 

 ion into utriculus and sacculus begins before any signs of the 

 canals appear. I find that in Amblystoma there is a distinction 

 of saccular region from that of the utriculus at the time the 

 Anlagen of the canals appear, but no division in the form of a 

 partition as described by Villy. In Amblystoma the separation 

 of utriculus from sacculus is concomitant with the later differ- 

 entiation of the canals. At the time the folds of the horizontal 

 and anterior canals have appeared (Fig. 8), there is seen in the 

 posterior part of the vesicle a fold extending obliquely down- 

 ward from the latero-dorsal wall (Fig. 9). This is to be consid- 

 ered as the lateral fold of the septum of the posterior canal. 

 Later this partition divides (Fig. 11), one part passing into the 

 septum of the posterior canal, and the other uniting with the 

 ventral fold of the septum of the horizontal canal. This parti- 

 tion is evidently homologous with the oblique partition described 

 by Villy in the frog, separating utriculus from sacculus. In 

 reality the partition at first in the frog as well as Amblystoma 

 does not separate utriculus from sacculus, but posterior canal 

 from horizontal canal. Later it becomes continuous with the 

 ventral fold of the septum of the horizontal canal, and it is the 

 latter fold that takes the greater share in the separation of utri- 

 culus from sacculus. The sacculus is constricted off by the 

 ingrowth of the folds above mentioned. As the posterior 

 oblique partition extends mesally it marks off the posterior 

 canal from the sacculus. The anterior portion of the ventral 

 fold of the septum of the horizontal canal also grows mesally 

 and separates utriculus from sacculus. The ingrowth of these 

 two folds continuous with each other leaves at length but a nar- 

 rowed opening between utriculus and sacculus, the utriculo-sac- 

 cular canal. As already pointed out, the first differentiation of 

 utriculus and sacculus is into anterior and posterior portions. 

 As the canals become distinct and the sacculus undergoes a 



1 Loc. cit. 



