Following fertilization, the fusion of two haploid germ cells, a diploid zygote with 46 chromosomes is produced. Mitotic division produces two daughter cells, each half the size of the original cell but containing the full complement of genetic information. Subsequent divisions produce a solid mass of cells by the fourth day ("morula"). As fluid accumulates within the morula, one group of cells accumulates on one side ("embryoblast") while the remaining group of cells surrounds the periphery ("trophoblast"). This new cystic structure is named a "blastocyst" (see Fig. 1.1). The outer trophoblast layer, from "tropho-" meaning nutrition, will become the placenta following implantation in the uterus by day 6. The inner embryoblast layer will become the embryo.