Final answer:
The period of prenatal development during which most of the body's structures and organ systems form is the period of the embryo.
Step-by-step explanation:
The period of prenatal development during which most of the body's structures and organ systems form is the period of the embryo. During weeks 3-8 of gestation, a developing human is referred to as an embryo. By the end of this period, all of the organ systems are structured in rudimentary form, although they are either nonfunctional or only semi-functional.
During the embryonic stage, cell division, migration, and differentiation occur. The heart begins to beat, organs form, and basic structures start to develop into areas that will become the head, chest, and abdomen. The neural tube also forms, which later develops into the spinal cord and brain.