An electron-microscopic study on the early development of the freshwater planarian Bdellocephala brunnea , with special reference to the formations of group and symcytium of yolk cells

The egg of freshwater planarians is a composite egg, composed of several egg cells and numerous yolk cells. The yolk cells around the egg cell surrounded the egg cell radiately, shortly after egg-laying. These yolk cells then formed the syncytium. Seilern-Aspang (1958) and Koscielski (1966) have assumed that the grouping substance released from the egg cell at 1-cell stage, and syncytial substance released from blastomeres. This time, we have examined these in embryo at 1- to 32-cell stage by electron microscopy. The cytoplasm of egg cell is filled with ER of vacuole type and free ribosome, and the ER vacuole are released outside of the egg. This is regarded to be closely related with grouping substance. After that, not only ER vacuoles but also plenty of vacuoles originated from mitochondria are released into the syncytium. We believe that these vacuoles are related with syncytial substance. The activities of these releases are high at 4- to 8-cell stages and it falls remarkably at the 16-cell stage. The lysosomes and autophagosomes that appear in the yolk cells probably bring about autodigestion of the cytoplasm.