DEVELOPMENTAL STUDIES ON THE LORICATE CHOANOFLAGELLATE STEPHANOECA-DIPLOCOSTATA ELLIS .8. NUCLEAR DIVISION AND CYTOKINESIS

In S. diplocostata the end of interphase coincides with the completion of costal strip production. The protoplast enlarges and the endomembr ane system, consisting of the Golgi apparatus and ER, is active in pro ducing fibrillar material which is stored in vesicles at the anterior end of the protoplast. Immediately prior to division the flagellum is withdrawn into the protoplast to one side of the flagellar base. The a xonematal microtubules are apparently severed in the transition region and the axoneme is withdrawn deep into the protoplast where it become s surrounded by the endomembrane system and is depolymerised. A second flagellar base appears beside the base of the resorbed flagellum. Bot h flagellar bases have a proximal ring of radial cytoskeletal microtub ules. At the beginning of nuclear division the anterior portion of the nuclear envelope disperses and, as one of the flagellar bases moves a way from the anterior end, the spindle microtubules appear. In a media n longitudinal section of the nucleus during metaphase the symmetrical arrangement of spindle microtubules between the two polar flagellar b ases is observed. Following nuclear division the two flagellar bases m igrate, one to the lower end of the parent protoplast and the other, w hich was at the anterior end, moves towards the equator. The proximal ring of cytoskeletal microtubules around each flagellar base is promin ent at this stage and it seems likely that the movement of the collar tentacles towards the equator is coordinated by these microtubules. Du ring telophase organelles are shared out between the developing daught er protoplasts. As the protoplast divides to produce two discrete daug hter protoplasts the two flagellar bases move to a central equatorial position and are inverted with respect to each other. The vesicles con taining fibrillar material remain within the anterior (juvenile) proto plast. Their ultimate disappearance coincides with lorica assembly and it is probable that the fibrillar material is used in the formation o f the lorica investment.