Final answer:
The germinal stage of prenatal development is marked by the division of the single-celled zygote.
Step-by-step explanation:
The germinal stage of prenatal development is marked by the division of the single-celled zygote. This stage lasts about two weeks and begins at conception when the egg and sperm cell unite to form a zygote. During this stage, the zygote undergoes rapid cell division, forming a solid ball of cells called a morula, and then a hollow ball of cells called a blastocyst.