DEVELOPMENT OF EAR AND ACOESSORY ORGANS IN FROG. 533 
the anterior end of the vesicle, near the point where the am- 
pulle of the two canals, with which it is destined to be asso- 
ciated, will be placed. A short time after the canals have been 
established this patch divides into two, of which one goes to 
form the sensory epithelium of each ampulla. 
The sensory epithelial areas of the cochlea and sacculus 
remain continuous with each other for some time. When the 
sacculus and lagena form, it follows from their position that 
both are lined by columnar epithelium, the former, however, 
only in part. The rapid growth of the sacculus necessitates 
the drawing out of the sensory lining of the floor of the vesicle, 
so that one end is now situated in the anterior part of the 
vesicle, and from this point it extends backwards to the end of 
the single cochlear pouch present, i.e. the lagena. The first 
part of the thickened epithelium to be separated from the rest 
lies on the under surface of the lip of the recessus, and when 
the pars neglecta of the cochlea develops it necessarily in- 
cludes it from its formation at the same point. The next 
sensory tract that differentiates is separated in the position 
where the remaining pouch of the cochlea (pars basilaris) 
will ultimately be, and forms the greater part of the wall 
pushed out in the development of this organ. The lining 
of the first formed division of the cochlea, i.e. the lagena, 
is the last to become distinct from the saccular epithelium. 
This fact may fairly be taken to support my view that the 
epithelial patches do not develop in the order in which they 
were evolved. 
I believe that it is of some importance to recognise that the 
outlying sensory tracts are individualised before the parts to 
which they are destined to belong. The ampullz and cochlear 
pouches are provided for in this way, all except the lagena; but 
this apparent exception does not really contradict the rule 
enunciated above, as the pouch under consideration is from the 
first placed in such a position that unless some secondary 
change were to take place it must be lined by sensory cells as 
soon as it is formed. The lagena appears early, when the 
canals have only just been established and when the epithelium 
VOL. XXX, PART 4,—NEW SER. MM 
