What is an embryo?

The correct definition of an embryo is a developing human organism from about 2 weeks after fertilization through the second month. This period marks significant developmental milestones, including the formation of major organs and structures. After fertilization, the zygote begins cell division and eventually travels down the fallopian tube, where it implants itself into the uterine lining. Once implantation occurs, the developing entity transitions into what is known as an embryo. This stage is crucial, as it represents early development and is characterized by rapid growth and differentiation of cells, leading to the establishment of foundational systems in the body.

The other options do not accurately capture this definition. A fertilized egg after implantation refers to a stage after the embryo has begun developing, which starts at the two-week mark. Describing it as a stage following the zygote but before birth fails to specify the important developmental characteristics and timeline associated with the embryonic stage. Lastly, identifying a fertilized egg before fertilization is incorrect as it misrepresents the events leading up to pregnancy.