During evolution from the genital pore to a simple oviduct (conduit) it is likely that the facilitating mechanism involved hormones acting as primary regulators at the cellular level. Adaptations to internal fertilization, egg processing, and megalecithal eggs required secondary regulator steroid hormones. Finally, viviparity and long gestation periods introduced extension of the role of secondary regulators and a third level of control via tertiary regulators tied to fetal development and maturation. Such a scheme accommodates known and as yet unknown hormone mechanisms and predicts the exceptions to generalizations that have troubled so many workers. The perspective presented here does not reduce the complexity of the system but rather orders the complexity to accommodate to an understanding of the process of parturition which given a common basis is obviously still not identical even for all viviparous mammals.