Editing Pregnancy (mammals) (section)

===After implantation===
A blastocele is a small cavity on the center of the embryo, and the developing embryonary cells will grow around it. Then, a flat layer cell forms on the exterior of this cavity, and the [[zona pellucida]], the blastocyst's barrier, remains the same size as before. Cells grow increasingly smaller to fit in. This new structure with a cavity in the center and the developing cells around it is known as a [[blastocyst]].

The presence of the blastocyst means that two types of cells are forming, an inner-cell mass growing on the interior of the blastocele and cells growing on the exterior of it. In 24 to 48 hours, the zona pellucida breaches. The cells on the exterior of the blastocyst begin excreting an enzyme which erodes [[epithelium|epithelial]] uterine lining and creates a site for implantation.