DEVELOPMENT OF THE EAR 719 



the tube and a subsequent dorsal dilatation, expanding as the tympanum 

 to include the auditory ossicles which have meanwhile taken form in the 

 adjacent mesenchyma. The mastoid cells are formed by a late erosion and 

 invasion of the bone by the mucous membrane of the tympanum. 



The internal ear develops from a thickening in the epidermal ectoderm 

 at the level of the third primary cerebral vesicle. This auditory anlage 

 becomes invaginated to form an otic pit, the aperture of which subse- 

 quently closes and thus separates an auditory vesicle or otocyst from the 

 overlying parent ectoderm. At the point of closure medially a dorsal 

 evagination arises to form the endolympJiatic duct. At the upper pole of 

 the vesicle appear the semicircular ducts through a process involving the 

 elevation of three circular folds, the lateral walls of which fuse proximally, 

 and subsequently disappear leaving a peripheral duct dilated at one end 

 to form an ampulla. The cochlea arises at the opposite pole as a tubular 

 evagination which becomes spirally disposed. The original vesicle persists 

 as the utricle; on its anteromedial border is formed an alveolar evagina- 

 tion, the saccule, which remains connected by a constricted duct, the utric- 

 ulosaccular canal, to which is attached the ductus endolymphaticus. 



Each ampulla differentiates an elongated patch of neuro-epithelium, 

 the cristce; in both utricle and saccule a similar oval patch appears, the 

 maculfE; the spiral organ of the cochlea develops in like manner through 

 a specialization of the ectoderm along the floor of the membranous duct. 



The bony labyrinth develops from the mesenchyma originally envelop- 

 ing the membranous labyrinth. The mesenchyma immediately surround- 

 ing the membranous labyrinth becomes converted into a mucoid tissue 

 which eventually disappears, leaving the perilymphatic spaces; these are 

 bounded by periosteum, the innermost layer of which becomes modified into 

 a mesenchymal epithelium. 



The scala tympani and scala vestibuli of the cochlea are formed by 

 the coalescence and subsequent dilatation of small mesenchymal tissue spaces 

 in two distinct regions: one between the saccule and the oval window, the 

 other between the saccule and the round window. From these two areas the 

 two great scalae of the cochlea proceed in a definite and constant direction 

 to their definite position and condition (Streeter, Proc. Amer. Assoc. Anat., 

 1916). 



