THE EAR. 143 



osseous walls of the auditory capsule are laid down. (Cf. 

 Figs. 75 and 68.) 



The accessory auditory apparatus. It will be convenient to 

 consider here the development of the Eustachian tube, and the 

 tympanic cavity and membrane, which, though only secondary 

 parts of the organ of hearing, are exceedingly characteristic of 

 terrestrial Vertebrates, as contrasted with the truly aquatic 

 Vertebrates, or Fishes. 



The Eustachian tube and tympanic cavity. The details of 

 development of these parts are not thoroughly determined. 



The Eustachian tube appears first in tadpoles of about 

 25 mm. length, as a solid rod of epithelial cells, running for- 

 wards from the anterior and dorsal edge of the first branchial 

 cleft. 



At the time of the metamorphosis, when the fore legs are 

 protruded, the Eustachian tube is a rod of cells with a very ill- 

 defined lumen, starting from the dorsal and anterior part of the 

 pharynx, and extending straight forwards beneath the eye ; it 

 is slightly dilated at its distal end, which lies opposite the 

 anterior border of the eye. 



During the metamorphosis, the Eustachian tube separates 

 from the pharynx, and divides into a variable number of short 

 lengths ; these gradually shift backwards to the position occu- 

 pied by the Eustachian tube in the adult frog ; by the time the 

 tail of the tadpole is completely absorbed, the several lengths 

 unite together, and with a diver ticulum from the pharynx, to 

 form the definite Eustachian tube of the adult, which now runs 

 almost directly outwards beneath the ear. The tympanic cavity 

 is merely the dilated outer end of this tube, lying just beneath 

 the surface ; and the layer of skin closing its outer end is the 

 tympanic membrane. (Of. Fig. 68, E, D.) 



From this account it appears that the tympanic cavity does 

 not at any period open on the surface of the head ; and it is 

 doubtful whether the Eustachian tube in the frog has any 

 definite relation to a gill-cleft. It is very probable that in this, 

 as in many other features of its embryological history, the frog 

 shows a modified rather than a primitive type of development. 



The tympanic cartilage. In tadpoles of about 40 mm. 

 length, shortly before the fore legs emerge, the tympanic carti- 

 lage appears as a dense mass of cells, surrounding the anterior 



