EMBRYO, ILLUSTRATION. EMBRYO, ILLUSTRATION The fertilized ovum or egg is composed of a unique cell, the zygote which contains the genetic material coming from both the spermatozoid and the female ovum. The fertilized egg divides several times forming a cell mass which implants in the uterine mucosa. Up to 8 weeks gestation, the in utero cell mass is called the embryo. At the 2 to 4 cell stage, embryos produced by in vitro fertilization can be implanted in a uterus or frozen for later implanta