The right answer is zygote ® blastocyst ® embryo ® fetus.
When a spermatozoon (sperm) joins the egg in one of the fallopian tubes, fertilization occurs; then begins cell division. The fertilized egg, or zygote, is transformed into a cellular mass called morula that goes down to the uterus. The zygote divides into two cells in two to three hours. After 5 days, it reaches about 90 cells. The division is continued until a blastocyst forms, a hollow mass consisting of a hundred cells.
A layer will cover it, called trophoblast, and begin to invade the uterine lining and form cords that attach the blastocyst to the lining of the uterus. The trophoblast will turn into a placenta, which is used to feed the embryo. Formed at the same time as the placenta, the umbilical cord connects the zygote to this one. At this point, the baby is called an embryo. It will be a fetus as soon as the organs have begun to develop.
