In human beings, the process of reproduction starts by the formation of male and female gametes respectively in the male and female reproductive systems. This is followed by the fusion of these gametes inside the female body, a process called fertilization. The fertilized product of this process is termed as a zygote. As zygote divides and travels up to the uterus, it becomes the embryo which gets implanted in the uterine walls. Thus, the correct sequence is gamete formation, fertilization, zygote, embryo.