By the time a mammalian female fetus reaches twelve weeks in gestation from the date of conception, it is already carrying within its fetal ovaries all of the primary oocytes that will, during puberty, years after its birth, become all of the eggs she will ovulate throughout her lifetime. During puberty, the brain begins to release GnRH, a hormone that stimulates the release of LH and FSH from the ovaries. (In males, GnRH released by the brain, stimulates the secretion of LH and FSH from the testes. These are the exact same hormones.) Each month, LH and FSH cause about 6-12 of the primary oocytes to mature. For some reason, one matures faster than the others and at ovulation, this one breaks through the surface of the ovary surrounded by a wispy mass called the cumulus. The vesicle that used to enclose it in the ovary remains behind as the corpus luteum, where it releases estrogen and progesterone that stop the release of LH and FSH. As a result of this drop in LH and FSH, no more oocytes mature and only one egg is ovulated per month. The ovulated egg floats in its cumulus until the fimbriae of the fallopian tube (which look like little fingers) pick it up and usher it toward the tube. If sperm are present, the egg will be fertilized here.

Fertilization is the process through which the genetic material from the sperm and the egg are combined to form one complete cell. Over the next day or two, cell division occurs while the zygote is being pushed and prodded down the fallopian tube toward the uterus. When the zygote is comprised of about 16 cells, we call it a morula - the latin word for mullberry. By the time it reaches the uterus, it contains about 120 cells and we call it a blastocyst. All the while, the trusty corpus luteum, way back in the ovary, has been releasing progesterone. As a result, the uterus builds up its lining of blood and nutrients in preparation for its coming guest. The blastocyst finds a comfortable spot in the upper segment of the uterus where it nuzzles itself into the lining. Here, the wild process of gastrulation takes place. Some of the undifferentiated cells of the blastocyst move to the center of the circle, like an "O" turning itself into a heart, and then a "U". The inside surface of this "U" becomes the digestive tract of the fetus; the top opening of the "U" - its anus.