Embryo and fetus are terms for a developing organism; they merely indicate different times during development. For humans, the embryo is an early stage of development, from approximately the fourth week after fertilization of an egg until about week 11. You will also see "embryo" refer to all early development from fertilization till week 11. Technically, the stages are fertilized egg (or zygote), blastula (a ball of undifferentiated cells), gastrula (the stage when indentation of the ball of cells occurs, and tissue layers form), and embryo. While the developing child is an embryo, its organs are forming and it begins to look somewhat human. At 11 weeks, most systems have begun to form; the developing human is now termed a fetus. This stage lasts until birth, and involves further growth and development.
Other animals go through this process, and the same terms are used. The time for each stage varies.
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