Editing Ontogeny (section)

===== Secondary Neurulation =====
In secondary neurulation, caudal and sacral regions of the spine are formed after primary neurulation is finished.  This process initiates once primary neurulation is finished and the posterior neuropore closes, so the tail bud can proliferate and condense, then create a cavity and fuse with the central canal of the neural tube.  Secondary neurulation occurs in the small region starting at the [[Tail bud|spinal tail bud]] up to the posterior neuropore, which is the open neural folds near the tail region that don't close through primary neurulation.  As canalization progresses over the next few weeks, neurons and ependymal cells (cells that create cerebral spinal fluid<ref>{{Cite web |title=Ependymal Cells |url=http://medcell.med.yale.edu/histology/nervous_system_lab/ependymal_cells.php |access-date=2022-04-10 |website=medcell.med.yale.edu}}</ref>) differentiate to become the tail end of the spinal cord.  Next, the closed neural tube contains neuroepithelial cells that immediately divide after closure and a second type of cell forms; the neuroblast.  [[Neuroblast]] cells form the mantle layer, which later becomes the [[Grey matter|gray matter]], which then gives rise to a marginal layer that becomes the [[white matter]] of the spinal cord.<ref name="Müller O'Rahilly Embryonic Development of the Central Nervous System"/>  Secondary neurulation is seen in the neural tube of the lumbar and tail vertebrae of [[frog]]s and [[Chicken|chicks]] and in both instances, this process is like a continuation of gastrulation.<ref name="Gilbert-2000" />
[[File:Acraea_zetes_caterpillar_to_pupae_to_butterfly_metamorphosis_by_Nick_Hobgood.jpg|thumb|Acraea zetes caterpillar to pupae to butterfly metamorphosis by Nick Hobgood]]