Final answer:
During the prenatal stage, a fertilized egg is initially called a zygote, which then forms into a morula and subsequently a blastocyst. Upon implantation in the uterus, it is called an embryo and after the eighth week of gestation, it becomes known as a fetus.
Step-by-step explanation:
During the prenatal stage, the fertilized egg is called a zygote immediately after conception. As the zygote travels through the fallopian tube towards the uterus, it divides by mitosis, forming a structure called a morula. This process known as cleavage continues without increasing the overall size of the organism.
Following this stage, the development progresses to the blastocyst stage, where the collection of cells forms into a blastocyst characterized by a fluid-filled cavity with an inner cell mass destined to become the embryo. The outer cell layer, known as the trophoblast, will develop into the chorionic sac and the fetal portion of the placenta.
Upon implantation in the uterine wall, typically 7 to 9 days after fertilization, this multi-cellular organism is then termed an embryo. The embryonic stage occurs from the third to the eighth week of development, during which all major organ systems begin to form. The embryo then transitions into a fetus. The fetal stage begins at the ninth week and lasts until birth, where the fetus undergoes growth and further maturation of organ systems.