Editing Animal embryonic development

{{Short description|Process by which the embryo forms and develops}}
{{About|embryonic development in all types of animals, including humans|information specific to  humans|Human embryonic development|information specific to plants|Plant embryonic development}}
[[File:Slack Essential Dev Biol Fig 02-08.jpg|thumb|Diagram of stages of embryo development to a [[larval]] and adult stage.]]

In [[developmental biology]], '''animal embryonic development''', also known as '''animal embryogenesis''', is the developmental stage of an [[animal]] [[embryo]]. Embryonic development starts with the [[fertilization]] of an [[egg cell]] (ovum) by a [[sperm|sperm cell]] ([[spermatozoon]]).<ref>{{cite book |last1=Gilbert |first1=Scott |title=Developmental Biology. 6th edition. Chapter 7 Fertilization: Beginning a new organism |date=2000 |url=https://www.ncbi.nlm.nih.gov/books/NBK10083/ |access-date=3 October 2020}}</ref> Once fertilized, the ovum becomes a single [[diploid cell]] known as a [[zygote]]. The zygote undergoes [[mitosis|mitotic]] [[cell division|division]]s with no significant growth (a process known as [[cleavage (embryo)|cleavage]]) and [[cellular differentiation]], leading to development of a multicellular embryo<ref name=Gilbert2>{{cite book |last1=Gilbert |first1=Scott |title=Developmental Biology. 6th edition. The Circle of Life: The Stages of Animal Development |date=2000 |url=https://www.ncbi.nlm.nih.gov/books/NBK9981/ |access-date=3 October 2020}}</ref> after passing through an organizational checkpoint during mid-embryogenesis.<ref>{{Cite journal|last1=Drost|first1=Hajk-Georg|last2=Janitza|first2=Philipp |last3=Grosse |first3=Ivo |last4=Quint |first4=Marcel | year=2017|title=Cross-kingdom comparison of the developmental hourglass|journal=Current Opinion in Genetics & Development|volume=45|pages=69–75|doi=10.1016/j.gde.2017.03.003|pmid=28347942|doi-access=free}}</ref>  In [[mammal]]s, the term refers chiefly to the early stages of [[prenatal development]], whereas the terms [[fetus]] and [[fetal development]] describe later stages.<ref name=Gilbert2/><ref>{{cite book |last1=Gilbert |first1=Scott |title=Developmental Biology. 6th edition. Early Mammalian Development |date=2000 |url=https://www.ncbi.nlm.nih.gov/books/NBK10052/ |access-date=3 October 2020}}</ref>

The main stages of animal embryonic development are as follows:
* The [[zygote]] undergoes a series of cell divisions (called cleavage) to form a structure called a morula.
* The morula develops into a structure called a blastula through a process called [[blastulation]].
* The blastula develops into a structure called a gastrula through a process called [[gastrulation]].
* The gastrula then undergoes further development, including the formation of organs ([[organogenesis]]).

The embryo then transforms into the next stage of development, the nature of which varies among different animal species (examples of possible next stages include a [[fetus]] and a [[larva]]).

==Fertilization and the zygote==
{{main|Zygote}}
The egg cell is generally asymmetric, having an [[animal pole]] (future [[ectoderm]]).
It is covered with protective envelopes, with different layers. The first envelope – the one in contact with the [[cell membrane|membrane]] of the egg – is made of [[glycoproteins]] and is known as the [[vitelline membrane]] ([[zona pellucida]] in [[mammals]]). Different [[taxa]] show different cellular and acellular envelopes englobing the vitelline membrane.<ref name=Gilbert2/><ref name="news-medical.net">{{cite news |last1=Hinton-Sheley |first1=Phoebe |title=Stages of Early Embryonic Development |url=https://www.news-medical.net/life-sciences/The-Stages-of-Early-Embryonic-Development.aspx |access-date=6 October 2020}}</ref>

[[Fertilization]] is the fusion of [[gametes]] to produce a new organism. In animals, the process involves a [[sperm]] fusing with an [[ovum]], which eventually leads to the development of an [[embryo]]. Depending on the animal species, the process can occur within the body of the female in internal fertilization, or outside in the case of external fertilization. The fertilized egg cell is known as the [[zygote]].<ref name=Gilbert2/><ref name="news-medical.net"/>

To prevent more than one sperm fertilizing the egg ([[polyspermy]]), fast block and slow block to polyspermy are used. Fast block, the membrane potential rapidly depolarizing and then returning to normal, happens immediately after an egg is fertilized by a single sperm. Slow block begins in the first few seconds after fertilization and is when the release of calcium causes the [[cortical reaction]], in which various enzymes are released from cortical granules in the eggs plasma membrane, causing the expansion and hardening of the outside membrane, preventing more sperm from entering.<ref>{{Cite web |last1=Alberts|first1=Bruce|last2=Johnson|first2=Alexander|last3=Lewis|first3=Julian|last4=Raff|first4=Martin|last5=Roberts|first5=Keith|last6=Walter|first6=Peter|date=2002|title=Fertilization|url=https://www.ncbi.nlm.nih.gov/books/NBK26843/|language=en|url-status=live|archive-url=https://web.archive.org/web/20170514114426/https://www.ncbi.nlm.nih.gov/books/NBK26843/|archive-date=2017-05-14}}</ref><ref name="news-medical.net"/>

== Cleavage and morula ==
{{See|Cleavage (embryo)}}

[[Image:Gray9.png|thumb|right|300px|Cell divisions (cleavage)]]

Cell division with no significant growth, producing a cluster of cells that is the same size as the original zygote, is called [[cleavage (embryo)|cleavage]]. At least four initial cell divisions occur, resulting in a dense ball of at least sixteen cells called the [[morula]]. In the early mouse embryo, the sister cells of each division remain connected during [[interphase]] by [[microtubule]] bridges.<ref>{{Cite journal |last1=Zenker |first1=J. |last2=White |first2=M. D. |last3=Templin |first3=R. M. |last4=Parton |first4=R. G. |last5=Thorn-Seshold |first5=O. |last6=Bissiere |first6=S. |last7=Plachta |first7=N. |date=September 2017 |title=A microtubule-organizing center directing intracellular transport in the early mouse embryo |journal=Science |language=en |volume=357 |issue=6354 |pages=925–928 |doi=10.1126/science.aam9335 |pmid=28860385 |bibcode=2017Sci...357..925Z |s2cid=206658036 |issn=0036-8075|doi-access=free }}</ref> The different cells derived from cleavage, up to the [[blastula|blastula stage]], are called [[blastomere]]s. Depending mostly on the amount of [[yolk]] in the egg, the [[cleavage (embryo)|cleavage]] can be [[cleavage (embryo)#Holoblastic|holoblastic]] (total) or [[cleavage (embryo)#Meroblastic|meroblastic]] (partial).<ref name=Campbell>[http://www.ncbi.nih.gov/About/primer/genetics_cell.html What is a cell?] {{webarchive|url=https://web.archive.org/web/20060118112245/http://www.ncbi.nih.gov/About/primer/genetics_cell.html |date=2006-01-18 }} 2004. A Science Primer: A Basic Introduction to the Science Underlying NCBI Resources. NCBI; and Campbell, Neil A.; Reece, Jane B.; '''Biology''' Benjamin Cummings, Pearson Education 2002.</ref><ref name=Gilbert3/>

Holoblastic cleavage occurs in animals with little yolk in their eggs,<ref>{{cite book |last1=Gilbert |first1=Scott |title=Early Development of the Nematode Caenorhabditis elegans |date=2000 |edition=Developmental Biology, 6th |url=https://www.ncbi.nlm.nih.gov/books/NBK10011/ |access-date=3 October 2020}}</ref> such as humans and other mammals who receive nourishment as embryos from the mother, via the [[placenta]] or [[milk]], such as might be secreted from a [[Pouch (marsupial)|marsupium]]. Meroblastic cleavage occurs in animals whose eggs have more yolk (i.e. birds and reptiles). Because cleavage is impeded in the [[vegetal pole]], there is an uneven distribution and size of cells, being more numerous and smaller at the animal pole of the zygote.<ref name=Campbell/><ref name=Gilbert3>{{cite book |last1=Gilbert |first1=Scott |title=Developmental Biology. 6th edition. An Introduction of Early Development Process |date=2000 |url=https://www.ncbi.nlm.nih.gov/books/NBK9992/ |access-date=3 October 2020}}</ref>

In holoblastic eggs, the first cleavage always occurs along the vegetal-animal axis of the egg, and the second cleavage is perpendicular to the first. From here the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms:

{| align="center" class="toccolours" border="1" style="border:1px solid gray; border-collapse:collapse;"
|+ '''Cleavage patterns followed by holoblastic and meroblastic eggs in animals'''
|-
!Holoblastic
!Meroblastic
|- valign="top"
|
* Radial ([[sea urchin]],<ref name=Gilbert3/><ref>{{cite book |last1=Gilbert |first1=Scott |title=Developmental Biology. 6th edition. The Early Development of Sea Urchins |date=2000 |url=https://www.ncbi.nlm.nih.gov/books/NBK9987/ |access-date=3 October 2020}}</ref> [[amphioxus]])
* Bilateral ([[tunicate]]s,<ref name=Gilbert3/><ref>{{cite book |last1=Gilbert |first1=Scott |title=Developmental Biology. 6th edition. Early Development in Tunicates |date=2000 |url=https://www.ncbi.nlm.nih.gov/books/NBK10017/ |access-date=4 October 2020}}</ref> [[amphibian]]s<ref>{{cite book |last1=Gilbert |first1=Scott |title=Developmental Biology. 6th edition. Early Amphibian Development |date=2000 |url=https://www.ncbi.nlm.nih.gov/books/NBK10113/ |access-date=3 October 2020}}</ref>)
* Spiral ([[annelid]]s, [[mollusk]]s<ref name=Gilbert3/><ref>{{cite book |last1=Gilbert |first1=Scott |title=Developmental Biology. 6th edition. The Early Development of Snails |date=2000 |url=https://www.ncbi.nlm.nih.gov/books/NBK10074/ |access-date=4 October 2020}}</ref>)
* Rotational ([[placental mammals]], [[marsupials]],<ref name=Gilbert3/><ref name=Gilbert4>{{cite book |last1=Gilbert |first1=Scott |title=Developmental Biology. 6th edition. Chapter 11. The early development of vertebrates: Fish, birds, and mammals |date=2000 |url=https://www.ncbi.nlm.nih.gov/books/NBK10062/ |access-date=3 October 2020}}</ref> [[nematode]]s<ref>{{cite book |last1=Gilbert |first1=Scott |title=Developmental Biology. 6th edition. Early Development of the Nematode Caenorhabditis elegans |date=2000 |url=https://www.ncbi.nlm.nih.gov/books/NBK10011/ |access-date=4 October 2020}}</ref>)
|
* Discoidal ([[fish]], [[monotremes]], [[bird]]s,<ref name=Gilbert3/><ref name=Gilbert4/> [[reptile]]s<ref name=Gilbert3/>)
* Superficial ([[insect]]s<ref name=Gilbert3/><ref>{{cite book |last1=Gilbert |first1=Scott |title=Developmental Biology. 6th edition. Early Drosophila Development |date=2000 |url=https://www.ncbi.nlm.nih.gov/books/NBK10081/ |access-date=4 October 2020}}</ref>)
|- valign="top"
|}

The end of cleavage is known as [[Midblastula|midblastula transition]] and coincides with the onset of zygotic [[DNA transcription|transcription]].

In amniotes, the cells of the [[morula]] are at first closely aggregated, but soon they become arranged into an outer or peripheral layer, the [[trophoblast]], which does not contribute to the formation of the embryo proper, and an [[inner cell mass]], from which the embryo is developed. Fluid collects between the trophoblast and the greater part of the inner cell-mass, and thus the morula is converted into a [[Vesicle (biology)|vesicle]], called the blastodermic vesicle. The inner cell mass remains in contact, however, with the trophoblast at one pole of the ovum; this is named the embryonic pole, since it indicates the location where the future embryo will develop.<ref name="bare_url">{{cite web|url=http://education.yahoo.com/reference/gray/subjects/subject/6|title=Yahoo|website=Yahoo|url-status=dead|archive-url=https://web.archive.org/web/20091222222223/http://education.yahoo.com/reference/gray/subjects/subject/6|archive-date=2009-12-22}}</ref><ref name=Gilbert3/>

== Formation of the blastula ==
{{See|Blastulation}}
After the seventh cleavage has produced 128 [[Cell (biology)|cell]]s, the morula becomes a [[blastula]].<ref name=Campbell/> The blastula is usually a spherical layer of cells (the [[blastoderm]]) surrounding a fluid-filled or yolk-filled cavity the [[blastocoel]].{{cn|date=October 2023}}

Mammals at this stage form a structure called the [[blastocyst]],{{ref|NCBI}} characterized by an [[inner cell mass]] that is distinct from the surrounding blastula.<ref>{{cite journal | vauthors = Nissen SB, Perera M, Gonzalez JM, Morgani SM, Jensen MH, Sneppen K, Brickman JM, Trusina A | display-authors = 6 | title = Four simple rules that are sufficient to generate the mammalian blastocyst | journal = PLOS Biology | volume = 15 | issue = 7 | pages = e2000737 | date = July 2017 | pmid = 28700688 | pmc = 5507476 | doi = 10.1371/journal.pbio.2000737 | doi-access = free }}</ref><ref name="What is the Blastocyst">{{cite web |last1=Balano |first1=Alex |title=What is the Blastocyst |url=https://sciencetrends.com/what-is-the-blastocyst/ |website=Science Trends |date=25 February 2019 |access-date=5 October 2020}}</ref><ref>{{cite web |title=Blastula |url=https://www.britannica.com/science/blastula |website=Encyclopedia Britannica |access-date=5 October 2020}}</ref> The blastocyst is similar in structure to the blastula but their cells have different fates.  In the mouse, primordial [[germ cell]]s arise from the inner cell mass (the [[epiblast]]) as a result of extensive [[genome]]-wide reprogramming.<ref name="pmid23223451">{{cite journal |vauthors=Hackett JA, Sengupta R, Zylicz JJ, Murakami K, Lee C, Down TA, Surani MA |title=Germline DNA demethylation dynamics and imprint erasure through 5-hydroxymethylcytosine |journal=Science |volume=339 |issue=6118 |pages=448–52 |date=January 2013 |pmid=23223451 |pmc=3847602 |doi=10.1126/science.1229277 |bibcode=2013Sci...339..448H }}</ref>  Reprogramming involves global [[DNA demethylation]] facilitated by the DNA [[base excision repair]] pathway as well as [[chromatin]] reorganization, and results in cellular [[Cell potency|totipotency]].<ref name="pmid20595612">{{cite journal |vauthors=Hajkova P, Jeffries SJ, Lee C, Miller N, Jackson SP, Surani MA |title=Genome-wide reprogramming in the mouse germ line entails the base excision repair pathway |journal=Science |volume=329 |issue=5987 |pages=78–82 |date=July 2010 |pmid=20595612 |pmc=3863715 |doi=10.1126/science.1187945 |bibcode=2010Sci...329...78H }}</ref><ref name="What is the Blastocyst"/>

Before [[gastrulation]], the cells of the trophoblast become differentiated into two layers: The outer layer forms a [[syncytium]] (i.e., a layer of protoplasm studded with nuclei, but showing no evidence of subdivision into cells), termed the [[syncytiotrophoblast]], while the inner layer, the [[cytotrophoblast]], consists of well-defined cells. As already stated, the cells of the trophoblast do not contribute to the formation of the embryo proper; they form the ectoderm of the [[chorion]] and play an important part in the development of the [[placenta]]. On the deep surface of the inner cell mass, a layer of flattened cells, called the [[endoderm]], is differentiated and quickly assumes the form of a small sac, called the [[yolk sac]]. Spaces appear between the remaining cells of the mass and, by the enlargement and coalescence of these spaces, a cavity called the [[amniotic cavity]] is gradually developed. The floor of this cavity is formed by the [[embryonic disk]], which is composed of a layer of prismatic cells – the embryonic ectoderm, derived from the inner cell mass and lying in apposition with the endoderm.<ref name="bare_url" /><ref name="What is the Blastocyst"/>

=== Formation of the germ layers ===
[[File:Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus (1953) (20661197932).jpg|thumb|Comparative vertebrate embryology.]]

The [[embryonic disc]] becomes oval and then pear-shaped, the wider end being directed forward. Towards the narrow, posterior end, an opaque [[primitive streak]], is formed and extends along the middle of the disc for about half of its length; at the anterior end of the streak there is a knob-like thickening termed the [[primitive node]] or knot, (known as ''Hensen's knot'' in birds). A shallow groove, the [[primitive groove]], appears on the surface of the streak, and the anterior end of this groove communicates by means of an aperture, the [[blastopore]], with the [[yolk sac]]. The primitive streak is produced by a thickening of the axial part of the ectoderm, the cells of which multiply, grow downward, and blend with those of the subjacent endoderm. From the sides of the primitive streak a third layer of cells, the [[mesoderm]], extends laterally between the ectoderm and endoderm; the [[Caudal (anatomical term)|caudal]] end of the primitive streak forms the [[cloacal membrane]]. The blastoderm now consists of three layers, an outer ectoderm, a middle mesoderm, and an inner endoderm; each has distinctive characteristics and gives rise to certain tissues of the body. For many mammals, it is sometime during formation of the germ layers that [[Implantation (human embryo)|implantation]] of the embryo in the [[uterus]] of the mother occurs.<ref name="bare_url" /><ref name="What is the Blastocyst"/>

== Formation of the gastrula ==
{{main|Gastrulation}}

During gastrulation cells migrate to the interior of the blastula, subsequently forming two (in [[diploblastic]] animals) or three ([[triploblastic]]) [[germ layer]]s. The embryo during this process is called a [[gastrula]]. The germ layers are referred to as the ectoderm, mesoderm and endoderm. In diploblastic animals only the ectoderm and the endoderm are present.<ref name=Campbell/>* Among different animals, different combinations of the following processes occur to place the cells in the interior of the embryo:
** [[Epiboly]] – expansion of one cell sheet over other cells{{ref|NCBI}}<ref name=Gilbert3/>
** Ingression – migration of individual cells into the embryo (cells move with [[pseudopods]]){{ref|NCBI}}<ref name=Gilbert3/>
** [[Invagination]] – infolding of cell sheet into embryo, forming the [[mouth]], [[anus]], and [[archenteron]].<ref name=Campbell/><ref name=Gilbert3/>
** Delamination – splitting or migration of one sheet into two sheets<ref name=Gilbert3/>
** Involution – inturning of cell sheet over the basal surface of an outer layer<ref name=Gilbert3/>
** Polar proliferation – Cells at the polar ends of the blastula/gastrula proliferate, mostly at the animal pole.<ref name=Gilbert3/>
* Other major changes during gastrulation:
** Heavy [[RNA transcription]] using embryonic genes; up to this point the [[RNA]]s used were maternal (stored in the unfertilized egg).
** Cells start major [[cellular differentiation|differentiation]] processes, losing their [[totipotent]]iality.

In most animals, a blastopore is formed at the point where cells are migrating inward. Two major groups of animals can be distinguished [[Embryological origins of the mouth and anus|according to the blastopore's fate]]. In [[deuterostome]]s the anus forms from the blastopore, while in [[protostome]]s it develops into the mouth.<ref name=Gilbert3/>

=== Formation of the early nervous system – neural groove, tube and notochord ===
{{Main|Development of the nervous system}}
In front of the primitive streak, two longitudinal ridges, caused by a folding up of the ectoderm, make their appearance, one on either side of the middle line formed by the streak. These are named the [[neural folds]]; they commence some little distance behind the [[anterior]] end of the [[embryonic disk]], where they are continuous with each other, and from there gradually extend backward, one on either side of the anterior end of the primitive streak. Between these folds is a shallow [[median]] groove, the [[neural groove]]. The groove gradually deepens as the neural folds become elevated, and ultimately the folds meet and coalesce in the middle line and convert the groove into a closed tube, the [[neural tube]] or canal, the ectodermal wall of which forms the rudiment of the nervous system. After the coalescence of the neural folds over the anterior end of the primitive streak, the blastopore no longer opens on the surface but into the closed canal of the neural tube, and thus a transitory communication, the [[neurenteric canal]], is established between the neural tube and the primitive [[digestive tube]]. The coalescence of the neural folds occurs first in the region of the [[hind brain]], and from there extends forward and backward; toward the end of the third week, the front opening ([[anterior neuropore]]) of the tube finally closes at the anterior end of the future [[brain]], and forms a recess that is in contact, for a time, with the overlying ectoderm; the hinder part of the neural groove presents for a time a [[rhombus|rhomboidal shape]], and to this expanded portion the term [[sinus rhomboidalis]] has been applied. Before the neural groove is closed, a ridge of ectodermal cells appears along the prominent margin of each neural fold; this is termed the [[neural crest]] or ganglion ridge, and from it the [[spinal nerves|spinal]] and [[human cranium|cranial]] nerve ganglia and the ganglia of the [[sympathetic nervous system]] are developed.{{Citation needed|date=December 2019|reason=removed citation to predatory publisher content}}  By the upward growth of the mesoderm, the neural tube is ultimately separated from the overlying ectoderm.<ref name="bare_url_a">{{cite web|url=http://education.yahoo.com/reference/gray/subjects/subject/7|title=The Neural Groove and Tube|website=Yahoo|archive-url=https://web.archive.org/web/20070822135819/http://education.yahoo.com/reference/gray/subjects/subject/7|archive-date=2007-08-22}}</ref><ref name=Gilbert3/>

[[File:Human embryo 8 weeks 4.JPG|thumb|250px|Dissection of human embryo]]

The [[cephalic]] end of the neural groove exhibits several dilatations that, when the tube is closed, assume the form of the three [[Brain vesicle|primary brain vesicle]]s, and correspond, respectively, to the future [[forebrain]] (prosencephalon), [[midbrain]] (mesencephalon), and [[hindbrain]] (rhombencephalon) (Fig. 18). The walls of the vesicles are developed into the nervous tissue and neuroglia of the brain, and their cavities are modified to form its ventricles. The remainder of the tube forms the [[spinal cord]] (medulla spinalis); from its ectodermal wall the nervous and neuroglial elements of the spinal cord are developed, while the cavity persists as the [[central canal]].<ref name="bare_url_a" /><ref name=Gilbert3/>

=== Formation of the early septum ===
The extension of the mesoderm takes place throughout the whole of the embryonic and extra-embryonic areas of the ovum, except in certain regions. One of these is seen immediately in front of the neural tube. Here the mesoderm extends forward in the form of two crescentic masses, which meet in the middle line so as to enclose behind them an area that is devoid of mesoderm. Over this area, the ectoderm and endoderm come into direct contact with each other and constitute a thin membrane, the [[buccopharyngeal membrane]], which forms a septum between the primitive mouth and [[pharynx]].<ref name="bare_url" /><ref name=Gilbert3/>

=== Early formation of the heart and other primitive structures ===
In front of the buccopharyngeal area, where the lateral crescents of mesoderm fuse in the middle line, the [[pericardium]] is afterward developed, and this region is therefore designated the pericardial area. A second region where the mesoderm is absent, at least for a time, is that immediately in front of the pericardial area. This is termed the proamniotic area, and is the region where the proamnion is developed; in humans, however, it appears that a proamnion is never formed. A third region is at the hind end of the embryo, where the ectoderm and endoderm come into apposition and form the cloacal membrane.<ref name="bare_url" /><ref name=Gilbert3/>

== Somitogenesis ==
{{main|Somitogenesis}}
[[File:Dynamic-3D-Cell-Rearrangements-Guided-by-a-Fibronectin-Matrix-Underlie-Somitogenesis-pone.0007429.s006.ogv|thumb|Sample 3D cell of somitogenesis|273x273px]]
Somitogenesis is the process by which [[somite]]s (primitive segments) are produced. These segmented tissue blocks differentiate into skeletal muscle, vertebrae, and dermis of all vertebrates.<ref name=Pourquié>{{cite journal |last1=Pourquié |first1=Oliver |title=Vertebrate Somitogenesis |journal=Annual Review of Cell and Developmental Biology  |date=November 2001 |volume=17 |pages=311–350 |doi=10.1146/annurev.cellbio.17.1.311 |pmid=11687492 |url=https://www.annualreviews.org/doi/abs/10.1146/annurev.cellbio.17.1.311 |access-date=5 October 2020|url-access=subscription }}</ref>

Somitogenesis begins with the formation of [[somitomeres]] (whorls of concentric mesoderm) marking the future somites in the presomitic mesoderm (unsegmented paraxial). The presomitic mesoderm gives rise to successive pairs of somites, identical in appearance that differentiate into the same cell types but the structures formed by the cells vary depending upon the anteroposterior (e.g., the [[thoracic]] vertebrae have ribs, the [[lumbar]] vertebrae do not). Somites have unique positional values along this axis and it is thought that these are specified by the [[Hox (gene)|Hox]] [[homeotic gene]]s.<ref name=Pourquié/>

Toward the end of the second week after fertilization, [[Transverse plane|transverse]] segmentation of the [[paraxial mesoderm]] begins, and it is converted into a series of well-defined, more or less cubical masses, also known as the somites, which occupy the entire length of the trunk on either side of the middle line from the [[:wikt:occipital|occipital]] region of the head. Each segment contains a central cavity (known as a [myocoel), which, however, is soon filled with angular and spindle-shape cells. The somites lie immediately under the ectoderm on the lateral aspect of the neural tube and [[notochord]], and are connected to the [[Lateral plate mesoderm|lateral mesoderm]] by the [[intermediate cell mass]]. Those of the trunk may be arranged in the following groups, viz.: [[neck|cervical]] 8, [[thoracic]] 12, [[lumbar]] 5, [[Sacrum|sacral]] 5, and [[coccygeal]] from 5 to 8. Those of the occipital region of the head are usually described as being four in number. In mammals, somites of the head can be recognized only in the occipital region, but a study of the lower vertebrates leads to the belief that they are present also in the anterior part of the head and that, altogether, nine segments are represented in the cephalic region.<ref>{{cite web|url=http://education.yahoo.com/reference/gray/subjects/subject/9|title=The Primitive Segments|website=Yahoo|archive-url=https://web.archive.org/web/20070911054326/http://education.yahoo.com/reference/gray/subjects/subject/9|archive-date=2007-09-11}}</ref><ref name=Pourquié/>

== Organogenesis ==
{{main|Organogenesis}}
[[File:Human embryo.jpg|thumb|Human embryo, 8–9 weeks, {{cvt|38|mm}}]]

At some point after the different germ layers are defined, [[organogenesis]] begins. The first stage in [[vertebrate]]s is called [[neurulation]], where the [[neural plate]] folds forming the neural tube (see above).<ref name=Campbell/> Other common organs or structures that arise at this time include the [[heart]] and somites (also above), but from now on embryogenesis follows no common pattern among the different taxa of the [[animal]]ia.<ref name=Gilbert2/>

In most animals organogenesis, along with [[morphogenesis]], results in a [[larva]]. The hatching of the larva, which must then undergo [[metamorphosis (biology)|metamorphosis]], marks the end of embryonic development.<ref name=Gilbert2/>

== See also ==
{{col div|colwidth=20em}}
* [[Cdx2]] gene
* [[Collective cell migration]]
* [[Drosophila embryogenesis|''Drosophila'' embryogenesis]]
* [[Enterocoely]]
* [[Homeobox]] genes
* [[Human embryogenesis]]
* [[Leech embryogenesis]]
* [[Parthenogenesis]]
* [[Plant embryogenesis]]
* [[Schizocoely]]
{{colend}}

== References ==

{{reflist|30em}}

== External links ==
* [http://www.scitopics.com/Cellular_Darwinism_stochastic_gene_expression_in_cell_differentiation_and_embryo_development.html Cellular Darwinism]
* [https://www.youtube.com/watch?v=Zjs9ya7A4MI Embryogenesis & MMPs], PMAP [[The Proteolysis Map]]-animation
* [http://www.ivanbelchev.com/myembryo Development of the embryo] (retrieved November 20, 2007)
* [https://www.doi.org/10.1111/j.1432-0436.2006.00114.x Video] of embryogenesis of the frog [[Xenopus laevis]] from shortly after [[fertilization]] until the hatching of the tadpole; acquired by [[MRI]] ([https://dx.doi.org/10.1111/j.1432-0436.2006.00114.x DOI of paper])

{{embryology}}
{{Developmental biology}}

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[[Category:Embryology]]
[[Category:Developmental biology]]