Editing Snake (section)

=== Embryonic development ===
[[File:Mouse_and_Snake_Embryos.jpg|thumb|right|[[Mouse]] embryo 12 day post [[Fertilisation|fertilization]] side by side with [[corn snake]] embryo 2 days post ovo-positioning.<ref name=":2" />]]
Snake embryonic development initially follows similar steps as any vertebrate [[embryo]]. The snake embryo begins as a [[zygote]], undergoes rapid cell division, forms a [[germinal disc]], also called a blastodisc, then undergoes [[gastrulation]], [[neurulation]], and [[organogenesis]].<ref name=":0">{{cite journal |last1=Zehr |first1=David R. |date=20 July 1962 |title=Stages in the Normal Development of the Common Garter Snake, Thamnophis sirtalis sirtalis |journal=[[Copeia]] |volume=1962 |issue=2 |pages=322–329 |doi=10.2307/1440898 |jstor=1440898}}</ref> Cell division and proliferation continues until an early snake embryo develops and the typical body shape of a snake can be observed.<ref name=":0" /> Multiple features differentiate the embryologic development of snakes from other vertebrates, two significant factors being the elongation of the body and the lack of limb development.{{Citation needed|date=November 2024}}
[[File:Comparative_somite_creation.jpg|thumb|Diagram illustrating differential [[somite]] size due to difference in [[somitogenesis]] clock oscillation<ref name=":2" />]]
The elongation in snake body is accompanied by a significant increase in [[vertebra]] count (mice have 60 vertebrae, whereas snakes may have over 300).<ref name=":2">{{cite journal |last1=Woltering |first1=Joost M. |title=From Lizard to Snake; Behind the Evolution of an Extreme Body Plan |journal=[[Current Genomics]] |year=2012 |volume=13 |issue=4 |pages=289–299 |doi=10.2174/138920212800793302 |pmid=23204918 |pmc=3394116}}</ref> This increase in vertebrae is due to an increase in [[somite]]s during embryogenesis, leading to an increased number of vertebrae which develop.<ref name=":2" /> Somites are formed at the [[Paraxial mesoderm|presomitic mesoderm]] due to a set of oscillatory genes that direct the [[Clock and wavefront model|somitogenesis clock]]. The snake somitogenesis clock operates at a frequency 4 times that of a mouse (after correction for developmental time), creating more somites, and therefore creating more vertebrae.<ref name=":2" /> This difference in clock speed is believed to be caused by differences in [[LFNG|''Lunatic fringe'' gene]] expression, a gene involved in the somitogenesis clock.<ref>{{Cite journal |last1=Gomez |first1=Céline |last2=Özbudak |first2=Ertuğrul M. |last3=Wunderlich |first3=Joshua |last4=Baumann |first4=Diana |last5=Lewis |first5=Julian |last6=Pourquié |first6=Olivier |date=July 17, 2008 |title=Control of segment number in vertebrate embryos |url=https://www.nature.com/articles/nature07020 |journal=[[Nature (journal)|Nature]] |language=en |volume=454 |issue=7202 |pages=335–339 |doi=10.1038/nature07020 |pmid=18563087 |bibcode=2008Natur.454..335G |s2cid=4373389 |issn=0028-0836 |access-date=30 April 2022 |archive-date=26 March 2023 |archive-url=https://web.archive.org/web/20230326063229/https://www.nature.com/articles/nature07020 |url-status=live }}</ref>

There is ample literature focusing on the limb development/lack of development in snake embryos and the gene expression associated with the different stages. In [[Henophidia|basal snakes]], such as the python, embryos in early development exhibit a hind [[limb bud]] that develops with some cartilage and a cartilaginous pelvic element, however this degenerates before hatching.<ref>{{cite journal |last1=Boughner |first1=Julia C. |last2=Buchtová |first2=Marcela |last3=Fu |first3=Katherine |last4=Diewert |first4=Virginia |last5=Hallgrímsson |first5=Benedikt |last6=Richman |first6=Joy M. |title=Embryonic development of Python sebae – I: Staging criteria and macroscopic skeletal morphogenesis of the head and limbs |journal=Zoology |date=June 2007 |volume=110 |issue=3 |pages=212–230 |doi=10.1016/j.zool.2007.01.005 |pmid=17499493 |bibcode=2007Zool..110..212B }}</ref> This presence of vestigial development suggests that some snakes are still undergoing hind limb reduction before they are eliminated.<ref name=":1">{{cite journal |last1=Leal |first1=Francisca |last2=Cohn |first2=Martin J. |date=January 2018 |title=Developmental, genetic, and genomic insights into the evolutionary loss of limbs in snakes |journal=[[Genesis (journal)|Genesis]] |volume=56 |issue=1 |pages=e23077 |doi=10.1002/dvg.23077 |pmid=29095557 |s2cid=4510082}}</ref> There is no evidence in basal snakes of forelimb rudiments and no examples of snake forelimb bud initiation in embryo, so little is known regarding the loss of this trait.<ref name=":1" /> Recent studies suggest that hind limb reduction could be due to mutations in enhancers for the [[Sonic hedgehog|SSH]] gene,<ref name=":1" /> however other studies suggested that mutations within the [[Hox gene|Hox Genes]] or their enhancers could contribute to snake limblessness.<ref name=":2" /> Since multiple studies have found evidence suggesting different genes played a role in the loss of limbs in snakes, it is likely that multiple gene mutations had an additive effect leading to limb loss in snakes<ref>{{Cite journal |last1=Kvon |first1=Evgeny Z. |last2=Kamneva |first2=Olga K. |last3=Melo |first3=Uirá S. |last4=Barozzi |first4=Iros |last5=Osterwalder |first5=Marco |last6=Mannion |first6=Brandon J. |last7=Tissières |first7=Virginie |last8=Pickle |first8=Catherine S. |last9=Plajzer-Frick |first9=Ingrid |last10=Lee |first10=Elizabeth A. |last11=Kato |first11=Momoe |date=October 2016 |title=Progressive Loss of Function in a Limb Enhancer during Snake Evolution |journal=[[Cell (journal)|Cell]] |language=en |volume=167 |issue=3 |pages=633–642.e11 |doi=10.1016/j.cell.2016.09.028 |pmc=5484524 |pmid=27768887}}</ref>