Mammals at this stage form a structure called the blastocyst, characterized by an inner cell mass that is distinct from the surrounding blastula. The blastocyst must not be confused with the blastula; even though they are similar in structure, their cells have different fates. In the mouse, primordial germ cells arise from a layer of cells in the inner cell mass of the blastocyst (the epiblast) as a result of extensive genome-wide reprogramming. Reprogramming involves global DNA demethylation facilitated by the DNA base excision repair pathway as well as chromatin reorganization, and results in cellular totipotency.