母川回帰シロサケの卵成熟過程

概要

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The process of oocyte maturation was observed in the chum-salmons, Oncorhynchus keta, captured at Otsuchi Bay and Otsuchi River, Iwate Prefecture, Japan, during the months of November 1969 and 1970. When the female salmons arrived at Otsuchi Bay, the oocytes conatined in their ovaries were either in the end of the growth period of oogenesis or in the beginning of the maturation period. From the time of arrival in the bay until the fish entered the river, a series of changes proceeded in the oocytes. The coalescence of yolk globules was first observed, followed by the migration and segregation of oil drops, the development of cortical cytoplasm, the formation of the blastodisc, the thickening of the chorion around the micropyle, the disappearence of the germinal vesicle, the meiotic division of the nucleus, and the rupture and shrinkage of the follicular envelopes. From the observation on oocyte structure, the maturation period of the oocytes can be devided into four stages, A, B, C and R. Because fish possessing fully ripe eggs in the body cavity were captured in the bay, it is ascertained that the oocytes can complete their maturation process within the bay prior to ascending the river.