A neurula is a vertebrate embryo at the early stage of development in which neurulation occurs. The neurula stage is preceded by the gastrula stage; consequentially, neurulation is preceded by gastrulation. Neurulation marks the beginning of the process of organogenesis. Mice, chicks, and frogs are common experimental models for studying the neurula. Depending on the species, embryos reach the neurula stage at different time points and spend a varying amount of time in this stage. For oviparous organisms, incubation temperature also affects the length of neurulation. In addition to development of the neural tube, other processes occur in a neurula stage embryo depending on the species. For example, in reptiles, extra-embryonic membrane tissues become distinct from the embryo. The neurula embryo has five regions of mesoderm that surround the neural tube. Anterior mesoderm develops into the head region, while posterior mesoderm develops into the trunk. Various molecules, including proteoglycans in the extracellular matrix, and genes, including Pax transcription factors, are essential for the development and closure of the neural tube in the neurula stage embryo. Neurulation Neurulation is a process in vertebrate embryos at the neurula stage in which the neural tube is formed. There are two types of neurulation: primary and secondary neurulation. Primary neurulation refers to the formation and inward folding of the neural plate upon itself to form the neural tube. In secondary neurulation, the neural tube forms via the merging of cavities in the medullary cord. In amphibians and reptiles, primary neurulation forms the whole neural tube, and the neural tube closes simultaneously along its length. Contrarily, in fish, secondary neurulation forms the neural tube. Both primary and secondary neurulation occur in birds and mammals, although with slight differences. Primary neurulation occurs in the cranial and upper spinal regions, which gives rise to the brain and upper regions of the spinal cord. Secondary neurulation occurs in the lower sacral and caudal regions, resulting in the formation of the lower regions of the spinal cord. In birds, the neural tube closes anterior to posterior, while in mammals, the middle closes first, followed by the closure of both ends. Developmental timing The point at which the embryo reaches the neurula stage differs among species, while for oviparous organisms, the length of neurulation is additionally affected by incubation temperature. In general, the lower the temperature, the greater the length of neurulation. Chick embryos reach the neurula stage on day