Phase 1 of the creation of life on earth was phase 2 was the formation of self-replicating, information-containing molecules. phase 3 was the development of a membrane, which enabled metabolism and created the first cells.

the corpus luteum secretes estrogens and progesterone. the latter hormone causes changes in the uterus that make it more suitable for implantation of the fertilized ovum and the nourishment of the embryo. if the egg is not fertilized, the corpus luteum becomes inactive after 10–14 days, and menstruation occurs.

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organ system-an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.