Human blastocyst within the zona pellucida

Human blastocyst contained within the zona pellucida, the coating that surrounds the egg and embryo up till this stage. It then hatches out of this membrane and implants into the uterine wall. The clump of cells within the blastocyst is the inner cell mass, which goes on to form the embryonic tissue. These cells can be collected and cultured to generate embryoniic stem cells (ES cells).