in the process can yield problems. For example, retained medullary cord occurs due to a partial or complete arrest of secondary neurulation that creates a non-functional portion on the vestigial end. Early brain development The anterior portion of the neural tube forms the three main parts of the brain: the forebrain (prosencephalon), midbrain (mesencephalon), and the hindbrain (rhombencephalon). These structures initially appear just after neural tube closure as bulges called brain vesicles in a pattern specified by anterior-posterior patterning genes, including Hox genes, other transcription factors such as Emx, Otx, and Pax genes, and secreted signaling factors such as fibroblast growth factors (FGFs) and Wnts. These brain vesicles further divide into subregions. The prosencephalon gives rise to the telencephalon and diencephalon, and the rhombencephalon generates the metencephalon and myelencephalon. The hindbrain, which is the evolutionarily most ancient part of the chordate brain, also divides into different segments called rhombomeres. The rhombomeres generate many of the most essential neural circuits needed for life, including those that control respiration and heart rate, and produce most of the cranial nerves. Neural crest cells form ganglia above each rhombomere. The early neural tube is primarily composed of the germinal neuroepithelium, later called the ventricular zone, which contains primary neural stem cells called radial glial cells and serves as the main source of neurons produced during brain development through the process of neurogenesis. Non-neural ectoderm tissue Paraxial mesoderm surrounding the notochord at the sides will develop into the somites (future muscles, bones, and contributes to the formation of limbs of the vertebrate ). Neural crest cells Masses of tissue called the neural crest that are located at the very edges of the lateral plates of the folding neural tube separate from the neural tube and migrate to become a variety of different but important cells. Neural crest cells will migrate through the embryo and will give rise to several cell populations, including pigment cells and the cells of the peripheral nervous system. Neural tube defects Failure of neurulation, especially failure of closure of the neural tube are among the most common and disabling birth defects in humans, occurring in roughly 1 in every 500 live births. Failure of the rostral end of the neural tube to close results in anencephaly, or lack of brain development, and is most often fatal. Failure of the caudal end of the neural tube to close causes a condition known