Destruction in mice of the adult S. mansoni after 2 months of infection, by treatment with an antimonial drug, has permitted establishing 32 days as the longevity of the egg in the tissues. Evidence indicates complete development of the miracidium at oviposition. With the passage of time, there is a gradual accumulation of excretory deposits within the egg, the amber color of the egg fades, and the miracidium becomes smaller and develops deformities. The flame cells of the miracidium continue to show activity until the death of the larva. These degenerative changes are similar to changes occurring in the egg recovered in the feces of the host.