6o6 COMPARATIVE ANATOMY 



Strengthened secondarily by the ingrowth of mesenchyme between the 

 two layers, forms the ear-drum. 



The three ear bones are primarily embedded in the connective tissue 

 dorsal to the tympanic cavity in which the spiracular pouch terminates. 

 This connective tissue subsequently disappears, and the tympanic cavity 

 is correspondingly enlarged and surrounds the earbones. The bones 

 thus become enclosed by the endodermic epithelium, which persists 

 throughout life. The origin of the three ear bones from elements of 

 the visceral skeleton is clearly evidenced by the facts of ontogenesis. 



The external auricle develops from materials supplied by the mandibu- 

 lar and hyoid arches. Three small hillocks arise on each of these arches, 

 and each hillock acquires a cartilaginous support. By the enlargement 

 and fusion of these six hillocks and their cartilaginous supports, the 

 external ear develops. The three nodules of the mandibular arch form 

 tragus and helix; those of the hyoid arch become antitragus and antihelix. 

 The claim that the cartilages of the ear are derived from those of the 

 hyoid arch has not been confirmed. 



