After fertilization, the zygote begins to divide rapidly. Twenty-four hours after fertilization, the zygote contains two cells, and three days after fertilization the zygote has 16 cells and is called a morula. The cells continue to divide and form a hollow bundle called a blastocyst. The blastocyst implants in the endometrium, and the cells begin to form the embryo. The embryo is present at five to six days after the zygote is fertilized. The embryo is a clump of cells inside the formerly hollow blastocyst, and is surrounded by the trophoblast.