As another completed phase of an inclusive study in the field of otological anatomy, the authors are now able to report on the morphogenesis of the tympanic ring and of certain related structures in man. Earlier articles in the series dealt with the following subjects: the vestibular fenestra (oval window) and the fissula ante fenestram;5,6 the otic capsule, the pericapsular tissues and the pneumatic spaces.v" the cochlear aqueduct and the periotic ductt" the tympanic wall of the lateral semicircular canal (that is, the region of surgical fenestration) ;2 the cranial aperture of the vestibular aqueduct and the related fovea for the endolymphatic (otic) sac;3,l1,18 the cochlear fenestra (round window) and secondary tympanic membrane.P and the auditory ossicles.1,4,17The present article will be devoted chiefly to consideration of the major steps in formation of the tympanic ring. Less detailed information will be presented on the concurrent development of the tympanic membrane, the tympanic cavity, the external acoustic meatus, and the squamous and petrous parts of the temporal bone.

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This description of a structure which, for the want of a better name, I have called the utriculo-endolymphatic valve, is based on the study of a large number of serial sections and reconstructions of human fetuses. Of the twenty-five