A mitotic zone has previously been demonstrated on the luminal surface of the posteromedial wall of the otocyst in the plane of the endolymphatic duct and at the junction of cochlea with primitive saccule. In this experiment the application of fluorodeoxyuridine (FudR) (an antimitotic agent) demonstrates the existence of a discrete synthetic zone for the otocyst which lies nearby to the otic zone of mitosis. Cells labeled by FudR and, therefore, in the mitotic synthetic (S) phase are always found away from the lumen (antiluminal surface) when the animals are sacrificed soon after injection. Cellular migration from synthetic zone through the luminal mitotic area and beyond is hypothesized. Once cellular division is completed the cells appear to follow a spiral course (from medial to lateral) over the anterior wall of the otocyst. The cells continue their spiral course entering the thin side of the cochlear base. From this strategic location the cells migrate around the sides of the cochlea to reach and to form the pseudostratified wall and complete the thin side of the cochlear duct. The pitch of this migration pattern is similar to the helical pitch of the mature cochlear duct and may be responsible for the adult configuration.