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4.   Describe the events of embryonic development from conception until the fetal stage, including development of the primary germ layers

embryonic period - Human prenatal development is divided into an embryonic period and a fetal period; the embryonic period begins with fertilization and ends eight weeks later; it involves the development of transient structures such as the three primary embryonic tissues which establish body plan, organogenesis and placental tissues, and their initial folding and form.; [Note:  some references define the first two weeks as the germinal period, which ends approximately 3-4 days after implantation and the embryonic period as the subsequent six weeks.]

embryo - The initial product of conception from implantation at about two weeks after fertilization through the eighth week of development.

fetal period - Human prenatal development is divided into an embryonic period and a fetal period; the second and longer fetal period (four times the embryonic period), from the ninth week to the fortieth week, approximately, or until delivery, is a time of extensive growth in size and mass as well as ongoing differentiation of organ systems established in the embryonic period.

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A fertilized egg (zygote) undergoes rapid mitotic cell division in a stage called cleavage. Cleavage provides the building blocks for constructing the embryo, which develops until it has about 100 cells and then hollows out to form a blastocyst. The blastocyst has two areas: the trophoblast and the inner cell mass. The inner cell mass forms the primary germ layers, which are the ectoderm, the endoderm, and the mesoderm. The ectoderm gives rise to the nervous system, the epidermis, and the skin. The endoderm forms the mucosae and associated glands. The mesoderm gives rise to everything else. The trophoblast develops projections called chorionic villi which form the placenta along with tissues from the motherʹs uterus. By the eighth week, all the organ systems have been laid down and the embryo looks distinctly human. Beginning in the ninth week, the embryo is referred to as a fetus.

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