When an egg is fertilised, normally within the fallopian tube, it divides into a ball of cells with a hollow cavity called a blastocyst.
The inner cell mass will become the develoing embryo and the outer layers of cells is the trophoblast that will give rise to the placenta.
About seven day after fertilisation the blastocyst implants into the wall of the womb, also known as the uterus, and produces root-like outgrowths called villi, which anchor the egg into the endometrial lining.
These villi grow into the placenta, which feeds and protects the baby until birth.
Implantation banner courtesy of Flickr under the creative commons licence: www.flickr.com/photos/lorelei-ranveig/2294096613/
Blastocyst image courtesy of Wikipedia under the GNU Free Documentation Licence: commons.wikimedia.org/wiki/File:Blastocyst.png