All life is made of cells, but all cells also have a shared history. Watch the video "Archaea and How DNA Began," and answer the questions that follow.

the dark variety, as they are not visible on the dark tree barks

explanation:

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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