three primary brain vesicles. four week embryo. prosencephalon or forebrain, mesencephalon or midbrain, rhombencephalon

developed structures embryonic brain 3d rendered medical illustration nervous system week embryo fetal development human formation branchial pharyngeal arches prenatal central neural neurodevelopment embryology concept umbilical cord structure tube science fetus developing spinal reproduction body anatomy medicine three primary vesicles four prosencephalon forebrain mesencephalon midbrain rhombencephalon hindbrain