 Thus, Aristotle could fit his observations of embryos into his larger theoretical interpretations of the world. In sexual generation, individual organisms begin when the fluids from the mother and the father come together. This combines the essential causes and initiates the developmental process. The maternal contribution is the material cause, which resides in the menstrual blood. After “the discharge is over and most of it has passed off, then what remains begins to take shape as a fetus” (Aristotle De Generatione Animalium [GA], 1.727b17–18). Yet the menstrual blood, or female semen, is only that out of which it generates and must be acted upon by the male semen which is that which generates. Together, consistent with the essential nature of the species and telos (or final cause) in question, the formal cause and efficient causal process act to bring a formed individual organism from potential into actual being. Gradually, over time, an individual organism’s form begins to emerge from the unformed. The male and female parents serve as the “principles of generation” for each individual organism (in sexually-generating species).