During the germinal period, villi emerge from the and then burrow into the uterine wall.

The germinal period is marked by the beginning of the conception wherein the sperm cell unites with the egg cell and forms a zygote.  During this stage, at about two weeks from fertilization, the zygote will drift down from the fallopian tube, will develop into an embryo, and will burrow itself into the uterine wall.  The embryo has a protective membrane called the chorion where tiny fingerlike villi, or minute blood vessels, emerge from in order to facilitate its attachment to the uterine lining.