Before dealing with maldevelopments of the external and middle ear, it is advisable to give a short account of the normal development of these parts. According to Young and Robinson, the visceral or pharyngeal clefts and arches are due to outward linear pouchings of the entoderm. The first cleft is the best marked. Its inner or medial part is converted into the eustachian tube and tympanum, while its outer or lateral part becomes the external auditory meatus. The membrane that lies at the bottom of the cleft consists at first merely of ectoderm and entoderm, but in a short time a thin layer of mesoderm is insinuated between the two and so the tympanic membrane is formed. The floor of the first cleft is thus transformed into the drumhead.
The margins of the visceral clefts are also thickened by the ingrowth of the mesoderm between the entodermal and ectodermal layers.
FRASER JS. MALDEVELOPMENTS OF THE AURICLE, EXTERNAL ACOUSTIC MEATUS AND MIDDLE EARMICROTIA AND CONGENITAL MEATAL ATRESIA. Arch Otolaryngol. 1931;13(1):1–27. doi:10.1001/archotol.1931.03660010011001