288 



THE DEVELOPMENT OF THE CHICK 



III. The Developmext of the Ear 



The ear develops from two entirely different primary sources, 

 viz., the otocyst,and the first visceral or hyomandibular cleft : The 

 former furnishes the epithelium of the membranous lab^-rinth; the 

 entodermal pouch of the latter becomes the tympano-eustachian 

 cavity; and part of the external furrow forms the external audi- 

 tory meatus; the tissue between the internal pouch and the ex- 

 ternal furrow develops into the tympanum. The mesenchyme in 

 the neighborhood of each of these primordia becomes modified, 



Fig. 166. — Reconstruction of the nasal cavity of a chick 



embryo of about 7 days; lateral view. (After Cohn.) 



Ch., Choanal, e. N., External nares. S. s'o., Supraor- 

 bital sinus. T. 1, T. 2, T. 3, Intermediate, superior and in- 

 ferior (vestibular) turbinals. 



(1) to form the bony labyrinth, perilymph, and other mesenchymal 

 parts of the internal ear, and (2) to form the auditory ossicles of 

 the middle ear. Thus the ear furnishes a striking example of the 

 combination of originally diverse components in the formation 

 of a single organ. The course of evolution of this complex sense- 

 organ is thus illustrated in the embryonic development; in the 

 Selachia the hyomandibular cleft is a communication between 

 pharynx and exterior, like the branchial clefts, and still preserves 

 to a certain extent the respiratory function. The embryonic 

 history furnishes a summary of the way in which it was gradually 



