The Placenta is an organ, unique to mammals, that begins to develop upon implantation of the blastocyst. It allows a developing foetus to absorb nutrients and oxygen from its mother's bloodstream prior to being born.

Young that grow this way are known as placental mammals.

Also see:
Zygote
Embryo
Foetus
Uterus
Gestation Period
Delayed Implantation
Fertilization
Sexual Reproduction