Neuromast Formation in the Prehatching Embryos of the Cod Fish, Gadus macrocephalus Tilesius

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To clarify when the free neuromasts of fish become functional, the neuromast formation in the embryonic development of the codfish Gadus macrocephalus Tilesius was investigated. The initial appearance of placodes and an increase in the number and distribution of free neuromasts were followed using light microscopy and scanning electron microscopy. Subsequently, the appearance of afferent and efferent nerve endings of the free neuromasts was investigated by transmission electron microscopy. Sections of embryos 72 hr before hatching revealed a pair of placodes on the head and two pairs on the trunk. The placodes increased in number, and some of them differentiated into free neuromasts during the embryonic stage. There were three pairs of free neuromasts on the head and four pairs on the trunk in newly hatched larvae. The afferent nerve endings were recognized first in the free neuromasts on the trunk at about 36 hr before hatching, and the efferent nerve endings were recognized on the trunk at about 24 hr before hatching. On the head, afferent nerve endings were seen at about 12 hr before hatching ; the efferent nerve endings were not yet seen in newly hatched larvae.
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