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coating, the egg immediately responds by both blocking out all other challengers and at the same
time pulling in the single successful sperm.
The Zygote
Within several hours, half of the 23 chromosomes from the egg and half of the 23 chromosomes
from the sperm fuse together, creating a zygote—a fertilized ovum. The zygote continues to
travel down the fallopian tube to the uterus. Although the uterus is only about 4 inches away in
the woman’s body, this is nevertheless a substantial journey for a microscopic organism, and
fewer than half of zygotes survive beyond this earliest stage of life. If the zygote is still viable
when it completes the journey, it will attach itself to the wall of the uterus, but if it is not, it will
be flushed out in the woman’s menstrual flow. During this time, the cells in the zygote continue
to divide: The original two cells become four, those four become eight, and so on, until there are
thousands (and eventually trillions) of cells. Soon the cells begin to differentiate, each taking on
a separate function. The earliest differentiation is between the cells on the inside of the zygote,
which will begin to form the developing human being, and the cells on the outside, which will
form the protective environment that will provide support for the new life throughout the
pregnancy.
The Embryo
Once the zygote attaches to the wall of the uterus, it is known as the embryo. During the
embryonic phase, which will last for the next 6 weeks, the major internal and external organs are
formed, each beginning at the microscopic level, with only a few cells. The changes in the
embryo’s appearance will continue rapidly from this point until birth.
While the inner layer of embryonic cells is busy forming the embryo itself, the outer layer is
forming the surrounding protective environment that will help the embryo survive the pregnancy.
This environment consists of three major structures: The amniotic sac is the fluid-filled reservoir
in which the embryo (soon to be known as a fetus) will live until birth, and which acts as both a
cushion against outside pressure and as a temperature regulator. Theplacenta is an organ that
allows the exchange of nutrients between the embryo and the mother, while at the same time
filtering out harmful material. The filtering occurs through a thin membrane that separates the