Following fertilization, there are rapid changes in the appearance of the Fucus egg. Large electron-translucent vesicles (V1) accumulate fibrillar material, and following pronuclear fusion, they are largely electron-opaque. These vesicles (V1) are formed originally in unfertilized eggs by smooth endoplasmic reticulum (SER) after release of the eggs from the oogonium. Golgi complex hypertrophy follows fertilization, and this increased activity continues throughout early embryogenesis. Wall formation begins after penetration of the egg by the sperm. Vesicles (V2) of unknown origin, which have homogeneously fibrillar contents, and Golgi vesicles (V3) merge with SER-derived vesicles (V1) after wall formation begins. Osmiophilic bodies are a prominent feature of the egg and embryo. They are penetrated by SER, and subsequently there is a loss of electron-opaque material. Alternatively, they discharge concentrically whorled material into the cytoplasm. The nuclear surface of the egg is convoluted in the period close to fertilization, and electron-opaque material is segregated in the cytoplasmic matrix lying within the nuclear invaginations.