Observation on the Embryo Development of Lutraria sieboldii Reeve(A Unique New Type of Cleavage — Fusion Cleavage)

The embryonic development process of Lutraria sieboldii Reeve was observed. The results showed that：(1) Like other bivalves, its embryonic development could be divided into 5 phases: cleavage, blastula, gastrula, trochophore and veliger. (2) In cleavage phase, the cleavage mode for Lutraria sieboldii Reeve was completely different from other animals. Although the ova of L. sieboldii Reeve obtained the ability to fertilize before they developed into the prophase of meiosis I (prior to the breakage of blastocyst), the release of polar bodies was not observed posterior to fertilization of ova, instead multi-nuclear fertilized ova were found. In later cleavage, longitudinal or latitudinal cleavages with the fertilized eggs did not occur as in other animals. A more unique and intricate type of cleavage, we called it fusion cleavage, was observed, viz., before 2-cell, 4-cell, 8-cell blastomeres entered into next division, nuclei disappeared gradually, blastomere fused gradually into one cell. After some time, in the centre of the cell appeared gradually nuclei that had been doubled in number. The nuclei gradually moved outward, and finally number-doubled blastomeres were cleaved out at one time.