Development begins when a sperm fertilizes an egg. 364
18.2 Development Before Birth
Development in humans involves certain processes and a set series of stages. 366The extraembryonic membranes and the placenta allow humans to develop internally within the uterus. 368Human development is divided into embryonic development, when all systems are established, and fetal development, when systems are completed. 372
18.3 Development of Male and Female Sex Organs
Male and female sex organs develop from the same indifferent tissue. 374
18.4 Birth
Birth is a multistage process that includes delivery of the child and the extraembryonic membranes. 376-77Lactation requires the hormone prolactin, and milk delivery requires the hormone oxytocin. 377
18.5 Development After Birth
Investigation into aging shows hope of identifying underlying causes of degeneration and prolonging the health span of individuals. 378