<P> In human pregnancy, a developing fetus is considered as an embryo until the ninth week, fertilization age, or eleventh - week gestational age . After this time the embryo is referred to as a fetus . </P> <P> First attested in English in the mid-14c., the word embryon derives from Medieval Latin embryo, itself from Greek ἔμβρυον (embruon), lit . "young one", which is the neuter of ἔμβρυος (embruos), lit . "growing in", from ἐν (en), "in" and βρύω (bruō), "swell, be full"; the proper Latinized form of the Greek term would be embryum . </P> <P> In animals, the development of the zygote into an embryo proceeds through specific recognizable stages of blastula, gastrula, and organogenesis . The blastula stage typically features a fluid - filled cavity, the blastocoel, surrounded by a sphere or sheet of cells, also called blastomeres . In a placental mammal, an ovum is fertilized in a fallopian tube through which it travels into the uterus . An embryo is called a fetus at a more advanced stage of development and up until birth or hatching . In humans, this is from the eleventh week of gestation . However, animals which develop in eggs outside the mother's body, are usually referred to as embryos throughout development; e.g. one would refer to a chick embryo, not a "chick fetus", even at later stages . </P> <P> During gastrulation the cells of the blastula undergo coordinated processes of cell division, invasion, and / or migration to form two (diploblastic) or three (triploblastic) tissue layers . In triploblastic organisms, the three germ layers are called endoderm, ectoderm, and mesoderm . The position and arrangement of the germ layers are highly species - specific, however, depending on the type of embryo produced . In vertebrates, a special population of embryonic cells called the neural crest has been proposed as a "fourth germ layer", and is thought to have been an important novelty in the evolution of head structures . </P>

Where does the young ones develop in humans