| Summary: | The embryogenesis and growth of the otoliths in the cod (Gadus morhua L.) was studied by light microscopy and transmission electron microscopy. The first indication of otolith formation was the appearance of electron-dense granules in the anterior and posterior parts of the otocysts at about the 30-somite stage of the embryos. The granules, which consisted of a condensed, flocculent material, were located close to the tips of the developlng kinocilia on the sensory cells and accumulated into a cluster, forming the core of the embryonic otoliths. A less condensed organic matrix was deposited around the otolith core, first segmentally, then in continuous layers. The organic matrix consisted of filamentous material, and isolated filaments could be observed in the endolymph around the developlng otoliths. The kinocilia remained in contact with the developlng otoliths for some days through canals in the deposited matrix, and as they increased in lenqth, they displaced the otoliths upward. Later, the distal part of the cilla disintegrated, but the canals persisted. In the lapilli (utricular otoliths) of newly hatched larvae reared under a 12L-12D photoperiod, 1-2 faint prehatch growth rings could occasionally be observed, but the first distinct growth ring was formed during the first 24 h after hatchlng and was separated from the central part of the otolith by a prominent boundary layer. Consecutive growth rings seemed to form every 24 h , but the rings were difficult to separate and could not be used for exact age determination. No distinct growth rings formed in the lapilli of larvae reared in constant darkness, and the otolith matrix little condensed, making these otoliths somewhat bigger and rounder than "normal" otoliths of he same age. No regular zone formation occurred in the lapilli of larvae reared in constant light. The distribution of precipitated calcium pyroantimonate in the otoliths suggested that the calcification of the otoliths started at hatching. Light microscopy of transverse sections through the otocysts of larvae showed that the lapillus was located over a horizontal sensory epithelium, whereas the posterior saccular otolith, the sagitta, was located near the medial wall of the otocyst, beside a vertical sensory epithelium. The lapillus was somewhat bigger than the sagitta at the early larval stage. Occasionally, a third body, which probably represented the developing third otolith, the asteriscus, could be observed in the dorsal part of the otocyst at the late embryonic stage.
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