Development of the ear in the common frog. 173 



the inner ear permanently from the portion of the canals within the 

 skull ; at the same time foramina are left, through which the com- 

 municating ducts pass. 



It seems most probable that the canals here described have some 

 connection with the conduction of sound. From their mode of 

 development and their permanent form, it is evident that they are 

 concerned with the cochlea, and it is very possible that the part 

 lying outside the skull above the pharynx receives vibrations, and 

 permits their passage to the auditory labyrinth. If this is the 

 function that the canals perform, they must have been employed in 

 this manner before the columella appeared, and still remained in use 

 after the terrestrial character and structure were acquired. 



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 possessing 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 by a 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 tad- 

 pole 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. 



