[Ooplasmic segregation and morphological axis formation in the polychaete Nereis virens embryo]

Ooplasmic segregation is of great importance in the development of Annelida. The mechanisms of this process are very diverse in different groups of polychaetes, oligochaetes, and leeches (Fernandez et al., 1998). Ooplasmic segregation in Nereis virens is connected with the first meiotic spindle formation and animal-vegetative axis appearance. Spherical polyaxial symmetry of the oocyte transforms into radial stratified symmetry in the course of ooplasmic segregation. There are two main steps of ooplasmic segregation in Nereis virens. The first step begins after the cortical reaction when the central clear cytoplasm reaches the surface of the oocyte. The movement of the cytoplasm is sensitive to nocodazole, colchicine, and cytochalasin B and appears to be mediated by microtubules and, partly, by microfilaments. The second step is not sensitive to the microtubule inhibitors and is mediated mainly by actin filaments. Ooplasmic segregation in Nereis virens may be considered as a primitive form of ooplasmic segregation in Annelida.