ИСТИНА

patterns and rules of morphogenesis mechanisms evolution. Among all cnidarians studied in the respect of embryonic development, the hydroid Dynamena pumila (Linnaeus, 1758) is exceptionally unusual. We described morphogenetic mechanisms of Dynamena early development using LM, CLSM and TEM. The expression pattern of the well known oral pole marker Wnt3 has been characterized at successive developmental stages. In the course of cleavage, Dynamena embryo becomes a morula, the loose aggregate of cell. In such morulae, gastrulation usually proceeds through the secondary delamination. In Dynamena, the gastrula stage is characterized by extremely irregular shape and very high level of morphological and morphogenetic variability. Secondary delamination in this species is based on the epithelization of the outer cells, which begins simultaneously in several regions of the embryonic surface leading to the formation of multiple blastoporelike “holes”. The "healing" of these holes provides the formation of a continuous epithelial sheet, and the hole that heals in last turn corresponds to the posterior pole of the planulalarva. Embryonic cells of Dynamena demonstrate very high migration activity. During in vitro rearing of embryos, their cells spread on the substrate until monolayer formation, but this does not prevent embryos from forming the normal body plan. We suppose that such a high level of cell migration activity provides massive cell rearrangements, which help Dynamena embryo to achieve a regular shape at the late gastrula stage.