Final answer:
An embryo becomes a foetus after approximately nine weeks of fertilization, marking the transition towards more advanced stages of development.
Step-by-step explanation:
An embryo becomes a foetus after approximately nine weeks following fertilization. During the embryonic stage, which lasts through the eighth week, the embryo grows and develops into a complex organism with structures such as the heart and early brain formation. By the end of the eighth week, the embryo undergoes significant development, with the beginnings of hands, feet, and facial features. The developmental milestone of transitioning from an embryo to a foetus marks a period wherein the organism starts to exhibit the recognizable form of a human being and continues to develop until birth.