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Explain embryo.

The term embryo applies to the earliest form of life, produced when an egg (female reproductive cell) is fertilized by a sperm (male reproductive cell; semen). The fertilized egg is called a zygote.
The zygote forms in one of the mother's fallopian tubes, the tubes that connect the ovaries with the uterus. It then travels to the uterus, where it becomes affixed to the uterine lining. Along the way, the zygote divides a number of times. By the time it reaches the uterus, it consists of about 100 cells and is called an embryoblast.

The exact day on which the embryoblast implants on the uterine wall varies, but is usually about the sixth day after fertilization. By the end of the first week, a protective sac, the amniotic cavity, begins to form around the embryoblast. Changes now begin to take place at a rapid rate.

During week two of embryonic development, embryonic cells have begun the process of differentiation. The identical cells formed by the early divisions of the zygote are beginning to take on the different characteristic of muscle, blood, nerve, bone, and other kinds of cells. The embryo has burrowed deep into the uterine wall and is visible as a bump on the inner uterine surface. This position permits the embryo to receive oxygen and nutrients from the mother's blood and to excrete waste products into her bloodstream.

The first three months of embryonic development are known as the first trimester, that is, the first three-month period of growth. At the end of the first trimester, the embryo looks like an adult, with all major organs having been formed. It is about 3 inches (7.5 centimeters) long.

At the beginning of the second trimester, the growing organism is no longer called an embryo, but a foetus.


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