embryo /em″brē·ō/ [Gk, en, in, bryein, to grow] , 1. any organism in the earliest stages of development. 2. in humans the stage of prenatal development from the time of fertilization of the ovum (conception) until the end of the eighth week. The period is characterized by rapid growth, differentiation of the major organ systems, and development of the main external features. Compare fetus, zygote. −embryonoid, embryonal, embryonic, adj.
