Implantation is a process in which a developing embryo ,moving
as a blastocyst through a uterus,makes contact with the uterine
wall and remains attached to it until birth.the lining of the
uterus prepares for the developing blastocyst to attach to it via
many internal changes.
The yolk sac is the structure vitally important during early
development. This highly vascular structure is surrounded by
nutrient-rich fluid and absorbs these nutrients and delivers them
for use by the embryo. This method of nutrition continues until the
placenta become more fully developed and begins functioning.