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. The embryo of a placental mammal is defined as the organism between the first division of the zygote (a fertilized ovum) until it becomes a fetus. An ovum is fertilized in a fallopian tube through which it travels into the uterus. In humans, the embryo is defined as the product of conception after it is implanted in the uterus wall through the eighth week of development. 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 eighth 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 late stages. 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. During organogenesis, molecular and cellular interactions between germ layers, combined with the cells' developmental potential, or competence to respond, prompt the further differentiation of organ-specific cell types.[citation needed] For example, in neurogenesis, a subpopulation of ectoderm cells is set aside to become the brain, spinal cord, and peripheral nerves. Modern developmental biology is extensively probing the molecular basis for every type of organogenesis, including angiogenesis (formation of new blood vessels from pre-existing ones), chondrogenesis (cartilage), myogenesis (muscle), osteogenesis (bone), and many others. Generally, if a structure pre-dates another structure in evolutionary terms, then it often appears earlier than the second in an embryo; this general observation is sometimes summarized by the phrase "ontogeny recapitulates phylogeny".[1] For example, the backbone is a common structure among all vertebrates such as fish, reptiles, and mammals, and the backbone also appears as one of the earliest structures laid out in all vertebrate embryos. The cerebrum in humans, which is the most sophisticated part of the brain, develops last. This sequencing rule is not absolute, but it is recognized as being partly applicable to development of the human embryo. Neural crest cells are a transient, multipotent, migratory cell population unique to vertebrates that gives rise to a diverse cell lineage including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia.[1] After gastrulation, neural crest cells are specified at the border of the neural plate and the non-neural ectoderm. During neurulation, the borders of the neural plate, also known as the neural folds, converge at the dorsal midline to form the neural tube. Subsequently, neural crest cells from the roof plate of the neural tube undergo an epithelial to mesenchymal transition, delaminating from the neuroepithelium and migrating through the periphery where they differentiate into varied cell types.[1] The emergence of neural crest was important in vertebrate evolution because many of its structural derivatives are defining features of the vertebrate clade.[2] Underlying the development of neural crest is a gene regulatory network, described as a set of interacting signals, transcription factors, and downstream effector genes that confer cell characteristics such as multipotency and migratory capabilities.[3] Understanding the molecular mechanisms of neural crest formation is important for our knowledge of human disease because of its contributions to multiple cell lineages. Abnormalities in neural crest development cause neurocristopathies, which include conditions such as frontonasal dysplasia, Waardenburg-Shah syndrome, and DiGeorge syndrome.[1] Therefore, defining the mechanisms of neural crest development may reveal key insights into vertebrate evolution and neurocristopathies.