The prenatal period is divided into three distinct stages, the germinal, the embryonic, and the fetal stages.
The Germinal Stage
. During this stage which begins with fertilization and ends when the ovum is embedded in the endometrium, the fertilized egg is referred to as a zygote. The duration of this stage is two weeks. The one cell zygote (the pronucleus of the egg has united with the pronucleus of the sperm to form one cell) at the end of the first has become a cluster of approximately 100 cells. Even at this early point the different cells are preparing to perform their individual tasks (differentiation). The external layer of the zygote serves as a protective skin and provides nourishment to the inner cells. The out cells (ectoderm) will later become the placenta, the umbilical cord, the Amniotic sac, etc.
MALE REPRODUCTIVE SYSTEM
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FEMALE REPRODUCTIVE SYSTEM
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The Embryonic Stage
. The second prenatal period begins at the time of implantment of the zygote in the endometrium and ends eight weeks later. During this stage 95% of the body parts have appeared through the process of cell differentiation.
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At the end of the first month three precise layers of cells have formed in the embryo (the zygote is now referred to as the embryo). Each of these layers serves a different function, the first becomes the ectoderm, the second the mesoderm, and the third the endoderm. At this time the embryo is about 1/4 of an inch long, which is about 10,000 times as large as the ovum that burst from the ovary. At the conclusion of this stage the embryo will be 1 1/4 inches long and weigh a whole 1/30th of an ounce.
The Fetal Stage
. This stage is best represented by the following chart. The stage extends from the 3rd month through the 9th month.
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