Developmental Cytology of Dictyosphaeria cavernosa. II. Nuclear Division during Zooid Formation

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Abstract
Mitosis in the coenocytic green alga, D. cavernosa, was studied with the electron microscope. In this alga mitosis and cytokinesis are separate events. The mitotic spindle is centric early in the course of zooid differentiation when many small cytoplasmic gaps appear in the peripheral cytoplasm. Prophase nuclei with centrioles are present only at this stage. Other stages of nuclear division are observed together in the network cytoplasm. After late prophase or prometaphase all centrioles are converted to the basal bodies with flagella and flagellar roots. Anaphase migration of chromosomes takes place at different rates and appears to be affected by inner membrane growth of the nuclear envelope to which chromosomes are attached, concomitant with this event a shortening of the chromosome-to-pole distance occurs and many long microtubules develop in the interzonal spindle. Separation of the interzonal spindle from daughter nuclei is accomplished by an invagination of inner nuclear membrane to which chromatin is attached. The nuclear envelope remains completely intact throughout mitosis.