THE SENSE ORGANS 425 



The intimacy of the connexion is gradually reduced during ontogenesis, 

 so that eventually the membrane loses its connexion with the cells of 

 the spiral organ and floats above them in the endolymphatic lluid. 



The differentiation of the elements of the spiral organ is not completed 

 at the time of birth, so that an infant is stone deaf until it is several weeks 

 old. 



The mesenchyma surrounding the membranous labyrinth becomes 

 differentiated into an inner connective-tissue membrane which lies in 

 contact with the epithelium of the sac, and an outer cartilaginous capsule 

 which encloses the membranous ear. Later, the portion of the cartilage 

 near the membranous sac disappears and is replaced by liquid perilymph 

 around the sac. As the cochlear duct develops, it becomes triangular in 

 cross section. 



In the higher vertebrates the cartilaginous otic capsule is subsecjuently 

 converted into bone by the process of endochondral bone formation. 

 But in the axis of the spiral cochlea the connective tissue is converted 

 directly into bone, after the manner of membrane-bone formation. The 

 contrast between the modes of formation of the inner and outer portions 

 of the bony part of the cochlea seems to have no phylogenetic significance. 



The middle ear and the Eustachian tube which leads from it to the 

 pharynx are both derived directly from the spiracular pouch, a diverticu- 

 lum of the pharynx. Consequently, both, like the pharynx from which 

 they evaginate, are lined by endoderm. To meet this endodermic diver- 

 ticulum, the ectoderm invaginates to form the external auditory meatus. 

 The two-layered membrane between the two invaginations, 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 ear bones. 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. 



