DEVELOPMENTAL STUDIES ON THE LORICATE CHOANOFLAGELLATE STEPHANOECA-DIPLOCOSTATA ELLIS .2. CELL-DIVISION AND LORICA ASSEMBLY

Cell division in Stephanoeca diplocostata follows the accumulation of a large number of costal strips in horizontal bundles at the top of the parent collar. Prior to nuclear division the flagellum is lost and the protoplast is large and rectangular. Nuclear division takes place whilst the protoplast undergoes vigorous metabolic movements and subsequent cytokinesis is achieved by equatorial constriction. The anterior of the two daughter protoplasts is the juvenile and is inverted with respect to the sister which remains attached to the parent lorica. The two protoplasts are joined by a cytoplasmic strand that consists of two threads both of which are initially attached to the daughter protoplasts at one side of the collar. Cell separation involves elongation of the strand and after each thread has broken contact with one of the daughter cells the two threads slide over each other until the juvenile is released. The juvenile takes the accumulation of supernumerary strips as it leaves the parent lorica and after release of the juvenile the strips are mobilised to form a new lorica. The collar tentacles of the parent are thought to play a significant role in the movement of strips during division and certain selected tentacles on the juvenile are associated with lorica assembly. Cell separation takes between 9-12 minutes and lorica assembly by the juvenile 2-3 minutes. © 1979 Springer-Verlag.