Time-lapse and electron microscopic studies of sea urchin micromeres

Summary The cleavage pattern of the young sea urchin embryo was studied by means of light and electron microscopy.The micromeres, which are known to have a strong organizing effect on the embryo, were found to form a syncytium with their neighbouring micromeres and with the macromeres. The cell walls between these cells were observed to be incomplete while there were interphase nuclei with intact nuclear membranes in the micro- and the macromeres. Similar phenomena with a break down of the cell membranes were not observed between macro- and mesomeres while there were intact interphase nuclei in these cells. Micromeres implanted on macromeres or mesomeres were found to coalesce with these latter cells in the course of a few minutes. During interphase, when the nuclei of both micro- and mesomere (macromere) had intact nuclear membranes, there also was a break down of the cell walls and a syncytium was formed by the host cell and the implanted micromere (see Fig. 6).The primary mesenchyme cells, which are regarded as the descendants of the micromeres, were also studied and were likewise found to form true syncytia.The importance to embryogenesis of this unique formation of syncytia is discussed.