Which one of following is initiated by secretion  of trophoblast?

The division of cells in an embryo is known as cleavage. Blastulation is the process of formation of blastula from morula and the process of formation of gastrula from blastula is known as gastrulation. During this process the three layers of the emberyo, the ectoderm, endoderm and the meoderm are fomed. During the formation of the blastocyst, the cells differentiate into the outer layer of cells surrounding a cavity called the blastocoel and an inner mass of cells. The embryo is implanted during this stage in the uterus. So, the correct option is 'Implantation'.