 The transition from the otic placode to the otic vesicle occurs during the 19th somite stage in Zebrafish, Xenopus, and chick.In chick, invagination of the otic placode occurs passively due to the movements of the surrounding placode.The otic placode in zebrafish, on the other hand, occurs by cavitation; the ectodermal placode condenses and forms an ovoid ball directly below the embryo surface.