Axenic cultures of Onoclea sensibilis L. form antheridia when treated with Antheridiogen Pt (A(,pt)). A dilute concentration of calcium hypochlorite, in contact with spores for less than 2 minutes, inhibits the rate of vegetative cell divisions, but has little affect on gametophyte morphology or antheridium development. The A(,pt) inhibits the rate of vegetative cell divisions to the same extent as calcium hypochlorite, but the effects are not synergistic;The maximum and minimum induction periods of antheridium formation are 5- 1/2 and 2- 1/2 days when treated on the day of sowing and 4 days later, respectively. The number of antheridia produced is highly correlated to the number of vegetative cells present per gametophyte;The division phase of antheridium development takes about 4 days. The differentiation of spermatids into spermatozoids takes about 3 days;First-formed antheridia occur on lateral, marginal cells and later-formed antheridia occur on upper, central cells. Only a portion of vegetative cells ever produce antheridia;Antheridium initials form at the anterior end of elongate, vegetative cells by an asymmetric cell division preceded by nuclear migration and an accumulation of RNA-staining cytoplasm in the anterior region. Prior to the first two cell divisions in the expanded initial, the cytoplasm becomes polarly distributed. A densely cytoplasmic, spermatogenous cell becomes enclosed by three vacuolate, jacket cells. The orientation of the divisions of the jacket cell walls is consistent with the classical theory of antheridium development. The daughter cells of the first two jacket cell divisions are different in shape, but similar in volume;Antheridia with 32 and 64 spermatids occur on gametophytes with equal frequency. The volume of each spermatid is similar regardless of the number present. The early stage in blepharoplast formation consists of a sphere of flocculent material containing two tri-laminate plaques oriented parallel to each other;During spermatid differentiation, two successive layers of cell wall material are deposited; the first layer during the rounding up of the spermatids, the second layer just before the spermatids decrease in volume and separate from the cell wall. Spermatozoids are released ensheathed in a layer of cell wall material.