of the neurula is formed by uniform expansion of the cells at the animal pole, known as the animal cap. The ectoderm then differentiates into neural and epidermal tissue. In reptilian embryos, beginning in the late-stage neurula and carrying over into the early stages of organogenesis, extra-embryonic membrane tissues comprising the yolk sac, chorion, and amnion become distinct from the tissues of the embryo. The mesoderm splits to create the extra-embryonic coelom, which consists of two layers. The vascularized mesoderm-endoderm inner layer, termed the splanchnopleure, develops into the yolk sac, while the nonvascularized ectoderm-mesoderm outer layer, termed the somatopleure, becomes the amnion and chorion. During organogenesis, these three extra-embryonic tissues become fully developed. Additionally, within the reptilian neurula, tissues of the brain begin to differentiate and the heart and blood vessels start to form. Chemical composition Mouse neurula tissues divide rapidly, with an average cell cycle lasting 8–10 hours. Proteoglycans in the extracellular matrix (ECM) of neurula-stage cells play an important role in promoting functional cranial neurulation and neural fold elevation; hyaluronic acid (HA) is synthesized and becomes accumulated, while the cell maintains a low level of sulfated glycosaminoglycans (GAGs). HA is involved in creation of biconvex neural folds, while sulfated GAGs are critical in manipulating the neural groove into a V-shape, as well as in neural tube closure. The ECM does not play a major role in spinal neurulation due to the close-packed nature of the mesodermal cells in the spinal region, which allows little intercellular space. Additionally, actin-containing microfilaments are believed to be necessary in cranial neurulation. They may act as the mechanism for neural folding, or they may stabilize neural folds that have already formed; however, their exact role has not been determined. There is some evidence that growth factors, such as insulin or transferrin, also play a role in neurulation, but this link has not been well-studied. Gene activation A variety of genes have been found to be expressed in the neurula stage embryo. Different genes are activated for different neurulation events, such as those occurring in separate regions of the developing neural tube. These genes are necessary for proper neurulation and closure of the neural tube. Signaling molecules such as Wnts, FGFs, and BMFs along with the transcription factors that include Msx, Snails, Sox8/9/10, and Pax3/7 genes play key roles in neural crest formation. Pax transcriptional factors have an important role in early development, especially