The acoelOtocelis luteola, when cultivated with either of two species ofNavicula, reaches maximum size and sexual maturity in 13 to 20 days. The proximal portion of the vagina and its opening are the first reproductive structures to appear. When a small invagination of the ventral epidermis forms the definitive genital pore and genital atrium, the opening of the vagina is displaced to the dorsal side of the atrium. The male copulatory organ, an eversible cirrus that will occupy part of the seminal vesicle and join the genital atrium, develops soon afterward, and the distal portion of the vagina, apparently without a lumen, becomes evident. After sperm have been formed, they are delivered by right and left channels to the seminal vesicle; a conspicuous sperm-filled space thus develops around the cirrus. The sclerotized nozzle of the seminal bursa is distinct by this stage, and enlarging oocytes soon become conspicuous. During final stages of differentiation, oocytes enlarge further. The granule-filled distal portion of the vagina arches over the seminal vesicle, reaching the seminal bursa. In a worm that has been inseminated, active sperm will be seen in one or more spaces within the bursa, which is otherwise rather vaguely delimited. If a mature specimen is starved, it regresses to a stage similar to that of one that has just hatched. Most reproductive structures are resorbed in an order inverse to that in which they differentiated. The cycle of differentiation and dedifferentiation has been repeated as many as seven times, and can perhaps be repeated indefinitely. Furthermore, some mature worms, after being isolated, remained normal for two years and were sexually competent when mated.