530 FRANCIS VILLY. 
ampulla itself, and this sensory epithelium is situated at the 
appropriate end of its canal. As the ampulle are formed 
they therefore include their special epithelial tracts from the 
very first. 
b. The Sacculus.—In tadpoles of about 11 mm. in length, 
just before any signs of the semicircular canals appear, the 
internal ear is complicated by the formation of a septum run- 
ning obliquely across its hinder part from the upper and outer 
to the inner and lower angle. This septum seems to grow 
forwards from the posterior wall of the vesicle, first appearing 
as a ridge and gradually broadening till it divides the pos- 
terior part of the vesicle into a larger upper and inner, and a 
smaller lower and outer part (see fig. 8). These two divi- 
sions of the vesicle communicate with the rest of the cavity in 
front, and therefore indirectly with each other. This septum 
is the first sign of a division into utriculus and sacculus. The 
upper portion of the vesicle becomes the hinder part of the 
utriculus in the adult, that is chiefly the posterior ampulla. 
The septum under consideration is at first confined to the 
posterior part of the vesicle and remains so for some time ; but 
later on it extends all round the vesicle, and growing inwards 
completes the division into utriculus and sacculus. This 
division is not quite complete, for the septum remains per- 
forated at its centre, thus leaving the utriculus and sacculus 
in communication. The septum consists of a double epiblastic 
fold for the most part. It attains the proportions of the adult 
in tadpoles of about 20 mm. 
Although the sacculus is formed in part merely by its sepa- 
ration from the rest of the vesicle by an incomplete septum, 
still it manifests active growth, and the great downwardly 
projecting pouch of the sacculus is mainly formed as an out- 
growth 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 
