Ultrastructural studies on early events in fertilization of the bivalveLaternula limicola

Summary Ultrastructural studies on sperm-egg interaction at the time of fertilization inLaternula limicola were performed. The temporary-acrosome did not change morphologically while the sperm passed through the egg investments. At the onset of sperm entrance into the egg, however, the temporary-acrosome and mitochondria were eliminated from the sperm. Afterwards the sperm was engulfed by the egg surface without membrane fusion of the gametes. After entry the sperm nucleus was surrounded by four membranes: the plasma membranes of the egg and of the sperm, and the membranes of the sperm nuclear envelope. As the sperm nucleus differentiated into the male pronucleus, the plasma membranes of both the sperm and egg were initially vesiculated, then dispersed into the egg cytoplasm. Finally, the sperm nuclear envelope changed into the male pronuclear membrane accompanying sperm chromatin dispersion.