Abstract
Thymic activity of the viviparous surfperch, Ditrema temmincki, was studied histologically throughout its life-span in relation to sexual maturity and gestation period.The materials used were caught on the coast of Aikawa Town, Sado Island in the Japan Sea, between June, 1978 and February, 1980.One hundred and sixty-six individuals were examined, including 123 adults, 16 juveniles and immatures and 27 fetuses.Males attain sexual maturity in August and begin mating.Mature eggs appear in November, and embryos, 1.5mm in total length, are seen in the following January.Since parturition occurs in June, the gestation period is suspected to be 6 months.The peak enlargement of the thymus in both sexes of adult individuals occurs in June with another lower peak appearing in October.There is a distinct difference in the thymic activity between the adult male and female in the early gestation period.From January to February, the thymic volume of females is remarkably larger than that of males.During this time, the thymus of the fetus is difficult to see due to its undifferentiated condition.The differentiation of fetal thymus begins in March when the fetus reaches 10mm in length.The mitotic figures of the large-and mediumsized lymphocytes are seen in this period.Toward April, small lymphocytes appear.The thymus of female fish is retained in an atrophic condition.However, a tremendous increase in its volume occurs in May and June, with a simultaneous increase in the thymic volume of the developing fetus.After parturition in June, the thymic volume of larval fish decreases month by month.In view of the relationship between the thymic volume of mother fish and fetus, the gestation period may be classified into three different periods: early (January to February), middle (March to April) and late (May to June).
