1. Seasonal changes in the ovary of the surf clam,
Spisula sachalinensis, collected in Yakumo, southern Hokkaido, were examined histologically. The process of the development of female germ cells was classified into (1) oogonium stage, (2) synaptic stage, (3) early yolkless stage, (4) late yolkless stage, (5) early yolk-formation stage, (6) late yolk-formation stage, and (7) maturation stage. Cytological characteristics of the female germ cells of each stage are described, especially concerning the formation of yolk and chorion in germ cells.
2. On the basis of cytological characteristics of female germ cells and of per cent ratio of oocytes of each stage, maturational processes of the ovaries were classified into (I) spent stage, (2) recovery stage, (3) early growing stage, (4) late growing stage, (5) maturing stage, and (6) spawning stage.
3. In the ovary, germ cells of the oogonium stage and of the synaptic stage are present embedded in the germinal epithelium throughout the year. In early October, oocytes of the yolkless stage begin to appear and grow gradually until December. Yolk formation in the oocytes begins in January, which causes a rapid growth of the ovary during the successive months. The ovary reaches its maximum size mostly in late April, but actual spawning is carried out during the period from late May to early June. The spawning is followed by a period of degeneration and disappearance of the remaining oocytes, which continues for one to two months, and then the ovary enters a period of gradual recovery for the next crop of oocytes.
4. An egg membrane appears at first at the place where the oocytes are protruded from the germinal epithelium, and no membrane can be found at the basal part of the oocyte buried in the germinal epithelium. At the early yolk-formation stage yolk globules make their appearance at first in the cytoplasm of the egg stalk and then are found distributed evenly throughout the whole cytoplasm of the oocyte.
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