primary neurulation begins in response to signals released by cells in the blastopore.
primary neurulation begins in response to signals released by cells in the?
blastopore
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.
neurula
each organism uses primary and secondary neurulation to varying degrees.





















































bullet::::- neurulation in "fish" proceeds only via the secondary form.





















































bullet::::- in "avian" species the posterior regions of the tube develop using secondary neurulation and the anterior regions develop by primary neurulation.





















































bullet::::- in "mammals", secondary neurulation begins around the 35th somite.





















































mammalian neural tubes close in the head in the opposite order that they close in the trunk.





















































bullet::::- in the head:





















































bullet::::- in the trunk:





















































section::::structure.
neural tube