DEVELOPMENT OF EAR AND ACCESSORY ORGANS IN FROG. 539 
This suggestion will not explain why the perilymphatic 
spaces should lie partially within the brain-case. I have 
imagined that this latter peculiarity may be due to their pos- 
sessing a secondary function of some description other than 
that already indicated. If this assumption is necessary, their 
complete meaning is still a problem, notwithstanding the 
partial solution already offered. 
The Eustachian Tube and Tympanic Cavity.—The 
development of these organs is marked bya peculiar and rapid 
change of position during the metamorphosis from the tadpole 
to the frog. The Eustachian tube at the beginning of the 
change is directed forwards and its end lies, not in connection 
with the ear, but under the anterior part of the eye below the 
palatopterygoid bar. As the tadpole loses its tail, the tube breaks 
up into short lengths which move backwards and come to lie 
in the position which they occupy in the adult. The isolated 
pieces then join themselves into a connected whole, which by 
gradual growth becomes the adult organ. Another point to be 
noted is that in development this tube has almost certainly 
nothing to do with the hyomandibular cleft. This mode of 
development in the lowest forms possessing such organs 
would seem at first sight to prove that the Eustachian tube is 
not morphologically equivalent to the hyomandibular cleft; but 
its development in the Anura is so peculiar that it must be 
regarded as highly modified, and no conclusions should be 
drawn too positively as to its history. As the Eustachian tube 
has generally been connected with the hyomandibular cleft in 
previous accounts of its development, I will here give a few 
notes on this cleft as found in the frog. 
In tadpoles of about 8 mm., the full number of clefts is 
already recognisable. They consist of five pairs of solid hypo- 
blastic outgrowths, the four posterior pairs having met and 
blended with the external epiblast, whilst the foremost or 
hyomandibular cleft ends in the mesoblast some distance from 
the exterior (see fig. 6). This stage represents the highest 
point in the development of the hyomandibular cleft, and after 
this period it commences to become smaller, never acquiring a 
