Updated by: Arabinda Naik
At the next stage of development, which produces an embryo with sixty-four cells, a cavity is formed within the cell mass. This cavity is called blastocoel and the embryo is called blastocyst. The embryo is called blastocyst which is composed of an outer envelope of cells, the trophoblast and an inner cell mass called embryoblast. The side of the blastocyst to which inner cell mass is attached is called the embryonic or animal pole while the opposite side is the abembryonic pole. The trophoblast encircles the blastocoel and inner cell mass. The inner cell mass gives rise to the embryo. The cells of the trophoblast help to provide nutrition to the embryo. They form the extra embryonic membranes, chorion and amnion and a part of the placenta. The cells of the trophoblast that are in contact with the inner cell mass are called cells of Rauber. As the blastocyst is formed, zona pellucida becomes thinner and finally disappears.