DEVELOPMENT OF BAR AND ACOESSORY ORGANS IN FROG. 527 
expanded into a ganglion of some size, whilst the nerve con- 
necting it with the brain consists at this stage of a fairly thick 
strand of cells. The whole is pigmented. During this period, 
or at least nearly the whole of it, until the tadpole is about 
11 mm. in length, the auditory nerve is absolutely continuous 
‘with the facial nerve. After this stage the ear becomes too 
complicated to describe clearly as a whole ; 1 propose, therefore, 
to take the various parts one-by one and follow them up to the 
adult condition in this order :— 
1. The Differentiation of the parts of the Internal Kar. 
The Semicireular Canals. 
The Sacculus. 
The Cochlea. 
. The Epithelium of the Auditory Vestibule. 
The Recessus Labyrinthi. 
f. The Perilymphatic Spaces. 
2. The Hyomandibular Cleft, Eustachian Tube, and 
Tympanic Cavity. 
3. The Annular Cartilage, the Stapes, and the Columella. 
SS euitee Goes 
a. The Semicircular Canals.—These canals are essen- 
tially parts of the auditory vesicle which have become separated 
from the main part by septa which are incomplete, inasmuch 
as each canal opens into the vesicle at each end of the septum. 
Each septum originally consists of two entirely distinct folds 
which grow together from opposite walls of the vesicle and by 
their coalescence cut off the canals except at their ends. The 
anterior vertical, and horizontal canals develop simultane- 
ously in the frog, whilst the posterior vertical is distinctly 
later in formation. 
The first indication of the canals is afforded at about 11 mm., 
by folds of the walls of the vesicles projecting internally. 
These folds are at first purely epiblastic; but very soon 
irregular mesoblastic cells migrate into the space included in 
each double fold. The ridges so formed are placed one at the 
anterior end, in the upper and inner part of the vesicle, and one 
opposite this and projecting towards it from the upper and 
