528 FRANOIS VILLY. 
outer part, and running back nearly to the hinder part of the 
ear. In its hinder half this latter projects ventrally instead of 
horizontally ; in fact, it is almost completely divided into two 
parts, the anterior corresponding to the first ridge mentioned 
and with it going to form the septum of the anterior vertical 
canal, whilst the posterior portion lies opposite to a third fold 
projecting upwards from the ventral part of the vesicle, these 
two constituting the septum of the horizontal canal. 
The two folds of each pair grow towards each other till they 
meet and coalesce for a considerable distance. At the stage 
now reached the anterior vertical and horizontal canals are 
already established. Figs. 7 and 8 show two stages in the 
formation of the canals as here described. In fig. 7 the septa 
have not yet been completed, whilst in fig. 8 the constituent 
folds have met and blended. Itshould be noted, however, that 
these two sections do not exactly correspond, for they cut the 
head at different levels. The septa formed in this way rapidly 
thicken and elongate, thus lengthening the canals, this length- 
ening keeping pace with the general growth of the vesicle 
during this period. 
Whilst these changes have been going on the posterior 
vertical canal has been forming. The first signs of its presence 
are apparent some time after the ridges forming the septa of 
the other canals have appeared, but before the coalescence of 
these ridges. It is formed exactly as the other canals are; the 
outgrowths developing, one on the septum between the sacculus 
and utriculus hereafter to be described, and situated on its outer 
edge directed upwards and inwards; the other is on the inner 
wall of the vesicle, more than half way up it and opposite to 
the first. This is shown in fig. 8. These folds coalesce after 
the time of cutting off of the two other canals. In the adult 
the posterior vertical canal curves over the horizontal canal and 
opens into the utriculus below it. This is brought about by 
the formation of the hinder ampulla, which by its growth 
causes an apparent inversion of these parts. The part of the 
utriculus originally above the horizontal canal develops into 
the ampulla and in so doing bulges out and encircles the canal. 
