Studies on the Normal Course of Development on Pelecypoda, Using Mainly the Eggs of C. gigas and C. chinensis
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概要
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The echinoderm eggs have been a well known object for the analysis of development (Yamamoto, 1988). This may be based mainly on their moderate size and transparency of the ovum. Moreover, the most important fact is that the eggs can be artificially inseminated.Hitherto, Yamamoto carried out experiments to learn the mechanism of syngamy(Yamamoto, 1973). As a matter of fact, the course of the syngamy of both pronucleiis known to be different in the kind of species. In the case of the sea urchin, bothpronuclei directly fuse together before the initiation of nuclear division. However, inthe eggs of certain molluscs, it is known that the direct fusion of both pronuclei doesnot occur prior to the formation of the mitotic spindle. In fact, the paternal andmaternal chromosomes become enclosed by common nuclear membranes in the nuclei of the two daughter cells (Balinsky, 1965).To learn the mechanism of syngamy in the latter cases, first of all, the writers tookup experiment to ascertain the normal development using some species of molluscs.In spite of their expectation, as it was difficult to pursue a course of cell linagebecause of the opacity of the eggs used, they intended to review the early developmentand larval differentiation of the Pelecypoda. In the present report, the writers described the normal course of development using mainly the eggs of the oyster, Crassostrea gigas and the bivalve, Caecera chinensis.Immediately after the shedding of the ovum, the unfertilized egg showed itself tobe polyhedral-shaped. Within 10 to 20 minutes after insemination, the breakdown ofthe germinal vesicle took place followed by the maturation division. In the case of C.gigas, just prior to the first cleavage, conspicuous bulging of the cytoplasm appearedat the vegetal side of the egg. When the cleavage occurred, this protuberance wasabsorbed into the one side of the daughter blastomere, and the egg was separatedinto the daughter cells of an unequal size. Continually, the writer first concentratedto pursue the cell linage, then to analyze the course of morphogenesis in the larvalstages of development.
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