Gametophytes of Pteridium aquilinum fed for six days with 50 and 75 p.p.m. thiouracil ceased to produce archegonia, but archegonia already initiated completed their development. Eggs produced in the presence of 50 p.p.m. thiouracil were viable, but embryogenesis was retarded. Gametophytes fed with 75 p.p.m. thiouracil mostly remained barren after insemination, the eggs being inviable, but others produced outgrowths from the archegoniate region which were either gametophytic, or sporophytic with a tendency to revert morphologically and functionally to gametophytic tissue.Thiouracil administered after fertilization either prevented embryogenesis, or retarded it, the sporophytes then being deformed and often showing a tendency to produce gametophytic tissue. The embryo became less sensitive to short periods of thiouracil as it developed, possibly because of the appearance of relatively quiescent regions, serving as reservoirs of unaffected cells.The results are held to support the view that the genes responsible for sporophytic growth become activated during oogenesis, and that the cytoplasm of the mature egg already contains the information leading to this kind of growth.