The Course of Mitosis in Young Embryos ofAscidiella Aspersa(Ascidiacea)

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Abstract
According to the morphology and behaviour of chromosomes and to the kinetic of the mitotic events, the embryonic mitosis of Ascidiella aspersa has been divided into 6 well distinguishable phases: prophase, prometaphase, metaphase, anaphase, anatelophase and telophase. Prometaphase is characterized by the contemporaneous appearance of the first fibrillar structures in the nuclear sap and by the sudden stretch of chromosome arms in the zone of the precociously separating daughter kinetochores. Anatelophase is characterized by chromosomes which undergo late anaphase movements contemporaneously with telophase alterations. The absence of nucleoli, and probably of nuclear envelope, is considered to be due to the extreme high speed of die blaslomere cleavage.