Embryologically the cochlea and their sensory cells develop and are completed at a relatively late stage of fetus compared with the vestibular organ (Fig. 2.1) [4]. The otocyst separates from the neural crest around gestation week 4, and the endolymphatic duct develops around gestation week 5. The membranous labyrinth develops into nearly the shape and size of the adult around gestation week 12, and the Corti’s organ is already complete at approximately gestation week 24 (Fig. 2.2). The whole size of the cochlea completes early at week 24 and does not change later (Fig. 2.3). In other words, the cochlea of a newborn has been morphologically completed by this time [1, 5]. This developmental plan in the cochlea suggests that inner ear malformation can be caused by arrest of process of cochlear development at very early gestation. Therefore, the reasons why there are many kinds of inner ear malformations must be caused by different times of arrest of fetus development.

Scarpa’s ganglion is located in the modiolus inside the cochlea. This ganglion markedly increases its size with progression of gestation. Its size at week 39 is fourfold of that at week 13 [5].

Jackler et al. presented a hypothesis of inner ear malformation from a view of development of inner ear in embryogenesis [6]: At the third gestational week, a failure of development would result in complete labyrinthine aplasia of the Michel type (Fig. 2.4).

A lack of normal differentiation beyond the fourth-week stage may result in the persistence of a large cloaca as seen in the common cavity cases: arrested development of the cochlear bud at this stage also would result in cochlear aplasia with preservation of the semicircular canals and vestibule.

A cessation of cochlear development during the sixth week may represent the various degrees of cochlear hypoplasia.

At the seventh week stage, an arrest of maturation may result in the classical Mondini’s deformity as a small cochlea with an incomplete intracochlear partition.

Other malformations may result from aberrant rather than arrested development or a combination of the two. This may account for unusual anomalies, such as extra-cochlear turns, cochlear duplication, and other such rarities.