Editing Ontogeny (section)

==== Neurulation ====
In developing [[vertebrate]] offspring, a [[neural tube]] is formed through either [[Neurulation|primary or secondary neurulation]].  Some species develop their spine and nervous system using both primary and secondary neurulation, while others use only primary or secondary neurulation.<ref name="Gilbert-2000">{{cite book |last1=Gilbert |first1=Scott F. |title=Developmental Biology |edition=6th |date=2000 |publisher=Sinauer Associates |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK10080/ |chapter=Formation of the Neural Tube }}</ref>  In human fetal development, primary neurulation occurs during weeks 3 and 4 of gestation to develop the brain and spinal cord.  Then during weeks 5 and 6 of gestation, secondary neurulation forms the lower sacral and coccygeal cord.<ref name="Müller O'Rahilly Embryonic Development of the Central Nervous System">{{cite book |doi=10.1016/B978-012547626-3/50003-X |chapter=Embryonic Development of the Central Nervous System |title=The Human Nervous System |date=2004 |last1=Müller |first1=Fabiola |last2=O'Rahilly |first2=Ronan |pages=22–48 |isbn=978-0-12-547626-3 }}</ref>

===== Primary Neurulation =====
The diagram to the right illustrates primary neurulation, which is the process of cells surrounding the neural plate interacting with neural plate cells to proliferate, converge, and pinch off to form a hollow tube above the [[notochord]] and mesoderm.  This process is discontinuous and can start at different points along the cranial-caudal axis necessary for it to close.<ref name="Müller O'Rahilly Embryonic Development of the Central Nervous System"/>  After the neural crest closes, the neural crest cells and ectoderm cells separate and the ectoderm becomes the epidermis surrounding this complex.  The neural crest cells differentiate to become components of most of the peripheral nervous system in animals.  Next, the notochord degenerates to become only the [[Intervertebral disc|nucleus pulposus]] of the [[intervertebral disc]]s and the mesoderm cells differentiate to become the [[somite]]s and skeletal muscle later on.  Also during this stage, the neural crest cells become the spinal ganglions, which function as the brain in organisms like [[earthworm]]s and [[arthropod]]s.<ref>{{Cite web |title=ganglion {{!}} physiology {{!}} Britannica |url=https://www.britannica.com/science/ganglion |access-date=2022-04-09 |website=www.britannica.com |language=en}}</ref>  In more advanced organisms like [[amphibian]]s, [[bird]]s and [[mammal]]s;<ref name="Gilbert-2000" /> the spinal ganglions consists of a cluster of nerve bodies positioned along the spinal cord at the dorsal and ventral roots of a spinal nerve, which is a pair of nerves that correspond to a vertebra of the spine.<ref>{{Cite web |title=spinal nerve {{!}} Definition, Function, Diagram, Number, & Facts {{!}} Britannica |url=https://www.britannica.com/science/spinal-nerve |access-date=2022-04-09 |website=www.britannica.com |language=en}}</ref>

===== 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]]