Final answer:
A human embryo transitions to a fetus at the end of the eighth week after fertilization, marking the start of the fetal stage of development.
Step-by-step explanation:
Technically, a human embryo becomes a fetus at the end of the eighth week after fertilization. This transition marks the end of the embryonic stage, a phase characterized by significant growth and changes including the formation of three cell layers, nervous system development, and the initial formation of most organs. Upon entering the fetus stage, starting the ninth week, the organism begins to take on a more recognizable form as a human being, with all organ systems structured in a rudimentary form although they are either nonfunctional or semi-functional at this stage.