Additionally, failure of the neural tube to close throughout the length of the body results in a condition called rachischisis. Patterning According to the French Flag model where stages of development are directed by gene product gradients, several genes are considered important for inducing patterns in the open neural plate, especially for the development of neurogenic placodes. These placodes first become evident histologically in the open neural plate. After sonic hedgehog (SHH) signalling from the notochord induces its formation, the floor plate of the incipient neural tube also secretes SHH. After closure, the neural tube forms a basal or floor plate and a roof or alar plate in response to the combined effects of SHH and factors including BMP4 secreted by the roof plate. The basal plate forms most of the ventral portion of the nervous system, including the motor portion of the spinal cord and brain stem; the alar plate forms the dorsal portions, devoted mostly to sensory processing. The dorsal epidermis expresses BMP4 and BMP7. The roof plate of the neural tube responds to those signals by expressing more BMP4 and other transforming growth factor beta (TGF-β) signals to form a dorsal/ventral gradient among the neural tube. The notochord expresses SHH. The floor plate responds to SHH by producing its own SHH and forming a gradient. These gradients allow for the differential expression of transcription factors. Complexities of the model Neural tube closure is not entirely understood. Closure of the neural tube varies by species. In mammals closure occurs by meeting at multiple points which then close up and down. In birds neural tube closure begins at one point of the midbrain and moves anteriorly and posteriorly. Secondary neurulation Primary neurulation develops into secondary neurulation when the caudal neuropore undergoes final closure. The cavity of the spinal cord extends into the neural cord. In secondary neurulation, the neural ectoderm and some cells from the endoderm form the medullary cord. The medullary cord condenses, separates and then forms cavities. These cavities then merge to form a single tube. Secondary neurulation occurs in the posterior section of most animals but it is better expressed in birds. Tubes from both primary and secondary neurulation eventually connect at around the sixth week of development. In humans, the mechanisms of secondary neurulation plays an important role given its impact on the proper formation of the human posterior spinal cord. Errors at any point