288 THE DEVELOPMENT OF THE CHICK 
Ill. THe DEVELOPMENT 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 labyrinth; the 
entodermal pouch of the latter becomes the tympano-eustachian 
eavity; 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, 
ia. 166. — Reconstruction of the nasal cavity of a chick 
embryo of about 7 days; lateral view. (After Cohn.) 
Ch., Choane. e. N., External nares. 38. 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 
