A cytomorphological study of Pelvetiopsis limitata (Setchell) Gardner confirmed a cytological alternation of generations in the plant. Approximately thirty-two chromosomes in the haploid phase alternate with sixty-four chromosomes in the diploid phase. Pelvetiopsis is oogamous and monoecious. Antheridia and oogonia are produced in flask-shaped conceptacles. Nuclear division is regular in developing gametangia. Reduction of chromosome number appears to take place during the first nuclear division in the antheridium and oogonium. No centrosomes or chromophilous spherules are discernible during nuclear division. In the oogonium the two divisions of the meiotic sequence are followed by a mitotic division producing eight nuclei. Two eggs of unequal size are formed in the oogonium. The larger egg is uninucleate and functional, while the smaller is seven-nucleate. At maturity 4 to 16 chromocenters may be observed in the nucleus of the large egg prior to its liberation. Before the release of the large egg from the oogonium approximately 32 chromosomes differentiate as the nucleus enters mitotic prophase. The smaller egg formed in the oogonium usually disintegrates after extrusion from the conceptacle. The inner layers of the wall of the oogonium are soon discarded after liberation. Aberrant eggs occur. Two eggs of equal size may form or there may be as many as 5 eggs of various sizes. Binucleate eggs are sometimes present. In the antheridium 4 mitotic divisions follow meiosis producing 64 nuclei. The cytoplasm differentiates around each nucleus and 64 spermatozoids are formed. These are released by the rupture of the outer layer, and by gelatinization of the inner layer of the wall of the antheridium. Approximately 30 - 32 chromosomes were observed at the second nuclear division in the antheridial initial. Oogamy in Pelvetiopsis restores the diploid complement of approximately 64 chromosomes. Fertilization occurs after the gametes have been liberated from the conceptacle. The egg is naked at the time of fertilization. The first division of the zygote is transverse to the plane in which the rhizoid develops. One, sometimes two, primary rhizoids form. Several other septate rhizoids develop later. The anatomy of the thallus of Pelvetiopsis is similar to other Fucaceae. Cryptostomata are rare and caecostomata are absent. Sterilized filtered seawater cooled to 6° to 9°C provides a suitable medium in which Pelvetiopsis limitata may be grown in culture. A minimum light intensity of 250 foot-candles promotes satisfactory development. / Science, Faculty of / Botany, Department of / Graduate