DEVELOPMENT OF EAR AND ACCESSORY ORGANS IN FROG. 5395 
stage is passed that this increase in size becomes most rapid and 
the blood-supply most copious. 
The relations of this organ at different stages are shown in 
figs. 4, 5, 7,9, 17, and 18. Its mode of origin and early stages 
have already been described, and until the semicircular canals 
are formed little is to be noted except a general growth in size, 
accompanied by a movement towards the brain so that it comes 
to lie in close contact with this organ. As the distal part 
comes close to the brain, it begins to expand and its duct 
narrows ; at the same time the upper lip of the duct elongates 
so as to carry the vestibular opening downwards. ‘The distal 
enlarged part grows, and, as the tadpole loses its tail, assumes 
the permanent proportions, becoming at the same time thin- 
walled and vascular, and the organs from the two sides meet 
both above and below the brain. Whether actual communica- 
tion is set up is difficult to determine by means of sections 
alone. 
The growth of cartilage between the expanded end of the 
organ and the rest of the vestibule does not take place till late, 
and even then a foramen is left through which the duct passes 
from the vestibule into the skull-cavity. 
The development of this structure shows that the whole of it 
is part of the internal ear and is not to be associated morpho- 
logically with the lymphatic spaces within the skull. No 
opening from the expanded portion within the skull is to be 
found connecting it with other cavities in any way. 
f. The Perilymphatic Spaces.—This system of canals 
has never been described adequately, although it is distinctly 
developed and very obvious in sections passing through the 
hinder part of the ear. Hasse and Retzius! have both given 
accounts of parts of these canals; but I believe that they 
have considered the two to be absolutely continuous with each 
other, and have described as one what really consists of two 
distinct parts. The canal which I have taken as the first is 
carefully described ; but the second seems to be taken as form- 
ing the anterior part of the first. Its passage into the skull 
1 «T), Gehororgan d. Wirbelthiere.’ 
