EMBRYONIC-DEVELOPMENT OF THE INNER-EAR AND OTOLITH OF THE RAINBOW-TROUT ONCORHYNCHUS-MYKISS

The embryonic development of the inner ear, especially the sensory epi thelia and otoliths in the rainbow trout Oncorhynchus mykiss, was stud ied by light and electron microscopy. Light microscopically, the audit ory vesicle, saccular otolith and statoacoustic ganglion were first ob served by 12 days after fertilization, while the utricular otolith app eared at 15 days after fertilization. Both the saccular and utricular maculae were more developed at 22 days after fertilization, and well d eveloped by 27 days, The crista ampullaris of the horizontal canal was also developed at 27 days after fertilization, while the other crista e were not yet distinguished. Electron microscopically, vesicular stru ctures and short microvilli were found on the sensory epithelia of the maculae by 15 days after fertilization. At 22 days after fertilizatio n, the saccular otolith possessed 7 incremental layers, and developing cilia, microvilli, and aggregates of secretory materials also appeare d on the apical surface of the sensory epithelia. At 27 days after fer tilization, the apical surface of each hair cell was covered with a ha ir bundle consisting of a single kinocilium and a bundle of stereocili a. These findings are discussed with special regard to the environment al factors on early development in fishes.