The spores of Ceratopteris thalictroides are large (104 × 128 μ), trilete, with a costate exine and devoid of perine. At germination a rhizoid is produced at the proximal pole, followed by a short uniseriate germ filament lateral to it. Formation of a prothallial plate is initiated by longitudinal divisions in the intercalary cells of the germ filament, and soon a strap-like plate is developed. The thallus broadens and becomes spatulate, but is devoid of any meristem. A pluricellular meristem is later differentiated from lateral marginal cells on one side of the thallus away from the anterior region. The meristematic region becomes notched and later cordate. The anterior half of the thallus expands and develops into a broad flat wing while a smaller, often uplifted wing is developed posterior to the meristem. A midrib is formed behind the meristem when the prothalli are ca. 5 weeks old. The mature prothallus is devoid of trichomes, asymmetrically cordate, with a large, spreading wing and a smaller, often curled and cornucopia-shaped wing lifted up from the substratum. The midrib is thin and bears rhizoids and archegonia on the lower surface. Antheridia are restricted to the wings and are common marginally; they are embedded in the prothallial tissue, and consist of a central mass of spermatozoids surrounded by a basket-shaped, thin, basal cell, a narrow ring cell, and a solitary disk-shaped cap cell. The latter opens like a lid at antheridial dehiscence. The first wall formed in the antheridial initial is basket-shaped and the second wall is nearly flat. Development of the embryo is of the common type in leptosporangiate ferns. The first juvenile leaf is naked, with a spatulate lamina supplied by a solitary vein, and associated with the first root. One or two thick multicellular hairs are produced at the base of the second leaf and, in the succeeding leaves, these appendages are palea-like. A stem apex becomes evident usually only after the fourth leaf is developed. All early juvenile leaves are subsessile with entire lamina. The venation becomes reticulate in the second or third juvenile leaf. It is concluded that Ceratopteris is more closely allied to Anemia and Mohria than to other genera of ferns.