Establishing a Loose Connection.
Apposition is the earliest contact between the blastocyst wall, the trophoblast and the endometrial epithelium.
This loose connection usually occurs between the crypt in the endometrium and the point in the blastocyst where the zona pellucida is ruptured most. Although ultimately the inner cell mass will be located closest to the deciduas layer by rotation within the blastocyst.
Implantation image courtesy of Flickr under the creative commons licence: www.flickr.com/photos/lorelei-ranveig/2294096613/