Final answer:
After fertilization, an egg cell is known as a zygote, which develops into a blastocyst before implantation, after which the internal cell mass is referred to as the embryo.
Step-by-step explanation:
Immediately after an egg cell has been fertilized, it is called a zygote. This single-cell structure results from the merging of a sperm and an egg at conception. As the zygote travels toward the uterus, it divides rapidly through a process known as cleavage, eventually forming into a morula and then a blastocyst. The blastocyst implants into the uterine wall around a week after fertilization, marking the beginning of pregnancy. The internal cell mass of the blastocyst will eventually develop into the embryo.