The nuclear conditions during the life-history of Fucus vesiculosus may be summarized as follows: 1. The Fucus plant contains 64 chromosomes and the number is reduced at the end of the first two nuclear divisions in the oogonium and antheridium initials. 2. Each of the four nuclei produced at the end of the first two divisions contains 32 univalent chromosomes, and this number persists up to the formation of the sperm and egg; the phase containing 32 chromosomes may be regarded as the gametophyte generation. 3. The union of the gametes doubles the number, and 64 chromosomes are present in every mitosis through the development of the Fucus plant up to the formation of the first four nuclei in the oogonium and antheridium initials. The phase containing 64 chromosomes may be regarded as the sporophyte generation. 4. There is thus present in Fucus an alternation of the gametophyte generation containing 32 chromosomes, with the sporophyte generation containing 64 chromosomes.