Editing Embryo (section)

===Animal embryos===
{{Main|Animal embryonic development}}
{{About|is a summary of embryonic development in all types of animals, including humans|information specific to  human embryonic development|Human embryonic development|section=yes}}
[[File:Embryonic development of a salamander, filmed in the 1920s.ogv|thumb|Embryonic development of salamander, circa the 1920s]]
[[File:Wrinkledfrog embryos.jpg|thumb|Embryos (and one [[tadpole]]) of the wrinkled frog (''Rana rugosa'')]]
[[File:Mouse and Snake Embryos.jpg|thumb|[[Mouse]] and [[snake]] embryos]]
In animals, fertilization begins the process of embryonic development with the creation of a zygote, a single cell resulting from the fusion of gametes (e.g. egg and sperm).<ref>{{Cite journal|url=https://opentextbc.ca/biology/chapter/24-6-fertilization-and-early-embryonic-development/|title=24.6. Fertilization and Early Embryonic Development – Concepts of Biology – 1st Canadian Edition|website=opentextbc.ca|date=14 May 2015|access-date=2019-10-30|last1=Molnar|first1=Charles|archive-date=2022-05-31|archive-url=https://web.archive.org/web/20220531143151/https://opentextbc.ca/biology/chapter/24-6-fertilization-and-early-embryonic-development/|url-status=live}}</ref> The development of a zygote into a multicellular embryo proceeds through a series of recognizable stages, often divided into cleavage, blastula, gastrulation, and organogenesis.<ref name=":0">{{Cite journal|last=Gilbert|first=Scott F.|date=2000|title=The Circle of Life: The Stages of Animal Development|url=https://www.ncbi.nlm.nih.gov/books/NBK9981/|journal=Developmental Biology. 6th Edition|language=en|access-date=2019-11-07|archive-date=2022-03-24|archive-url=https://web.archive.org/web/20220324172948/https://www.ncbi.nlm.nih.gov/books/NBK9981/|url-status=live}}</ref>

Cleavage is the period of rapid mitotic cell divisions that occur after fertilization. During cleavage, the overall size of the embryo does not change, but the size of individual cells decrease rapidly as they divide to increase the total number of cells.<ref>{{Cite web|url=http://11e.devbio.com/wt0102.html|title=DevBio 11e|website=11e.devbio.com|access-date=2019-11-07|archive-date=2019-10-30|archive-url=https://web.archive.org/web/20191030192621/http://11e.devbio.com/wt0102.html|url-status=live}}</ref> Cleavage results in a blastula.<ref name=":0" />

Depending on the species, a blastula or [[blastocyst]] stage embryo can appear as a ball of cells on top of yolk, or as a hollow sphere of cells surrounding a [[blastocoel|middle cavity]].<ref name=":1">{{Cite book|title=An Introduction to Embryology|last=Balinsky|first=Boris Ivan|publisher=W.B. Saunders Company|year=1975|isbn=0-7216-1518-X|edition=Fourth}}</ref> The embryo's cells continue to divide and increase in number, while molecules within the cells such as RNAs and proteins actively promote key developmental processes such as gene expression, cell fate specification, and polarity.<ref>{{Cite journal|last=Heasman|first=Janet|date=2006-04-01|title=Patterning the early Xenopus embryo|journal=Development|language=en|volume=133|issue=7|pages=1205–1217|doi=10.1242/dev.02304|issn=0950-1991|pmid=16527985|doi-access=free}}</ref> Before [[Implantation (embryology)|implanting]] into the uterine wall the embryo is sometimes known as the ''pre-implantation embryo'' or ''pre-implantation conceptus''.<ref name="Niakan">{{cite journal |last1=Niakan |first1=KK |last2=Han |first2=J |last3=Pedersen |first3=RA |last4=Simon |first4=C |last5=Pera |first5=RA |title=Human pre-implantation embryo development. |journal=Development |date=March 2012 |volume=139 |issue=5 |pages=829–41 |doi=10.1242/dev.060426 |pmid=22318624|pmc=3274351 }}</ref> Sometimes this is called the '''pre-embryo''' a term employed to differentiate from an embryo proper in relation to embryonic stem cell discourses.<ref name="Jones">{{cite journal |last1=Jones |first1=DG |last2=Telfer |first2=B |title=Before I was an embryo, I was a pre-embryo: or was I? |journal=Bioethics |date=January 1995 |volume=9 |issue=1 |pages=32–49 |doi=10.1111/j.1467-8519.1995.tb00299.x |pmid=11653031}}</ref>

Gastrulation is the next phase of embryonic development, and involves the development of two or more layers of cells (germinal layers). Animals that form two layers (such as [[Cnidaria]]) are called diploblastic, and those that form three (most other animals, from [[flatworm]]s to humans) are called triploblastic. During gastrulation of triploblastic animals, the three germinal layers that form are called the [[ectoderm]], [[mesoderm]], and [[endoderm]].<ref name=":1" /> All tissues and organs of a mature animal can trace their origin back to one of these layers.<ref>{{Cite journal|last1=Favarolo|first1=María Belén|last2=López|first2=Silvia L.|date=2018-12-01|title=Notch signaling in the division of germ layers in bilaterian embryos|journal=Mechanisms of Development|volume=154|pages=122–144|doi=10.1016/j.mod.2018.06.005|pmid=29940277|issn=0925-4773|doi-access=free|hdl=11336/90473|hdl-access=free}}</ref> For example, the ectoderm will give rise to the skin epidermis and the nervous system,<ref>{{Cite web|url=https://embryo.asu.edu/pages/ectoderm|title=Ectoderm {{!}} The Embryo Project Encyclopedia|website=embryo.asu.edu|language=en|access-date=2019-11-07}}</ref> the mesoderm will give rise to the vascular system, muscles, bone, and connective tissues,<ref>{{Cite web|url=https://embryo.asu.edu/pages/mesoderm|title=Mesoderm {{!}} The Embryo Project Encyclopedia|website=embryo.asu.edu|language=en|access-date=2019-11-07|archive-date=2024-09-10|archive-url=https://web.archive.org/web/20240910221203/https://embryo.asu.edu/pages/mesoderm|url-status=live}}</ref> and the endoderm will give rise to organs of the digestive system and [[epithelium]] of the digestive system and respiratory system.<ref>{{Cite journal|last1=Zorn|first1=Aaron M.|last2=Wells|first2=James M.|date=2009|title=Vertebrate Endoderm Development and Organ Formation|journal=Annual Review of Cell and Developmental Biology|volume=25|pages=221–251|doi=10.1146/annurev.cellbio.042308.113344|issn=1081-0706|pmc=2861293|pmid=19575677}}</ref><ref>{{Cite journal|last1=Nowotschin|first1=Sonja|last2=Hadjantonakis|first2=Anna-Katerina|last3=Campbell|first3=Kyra|date=2019-06-01|title=The endoderm: a divergent cell lineage with many commonalities|journal=Development|language=en|volume=146|issue=11|pages=dev150920|doi=10.1242/dev.150920|issn=0950-1991|pmid=31160415|pmc=6589075}}</ref> Many visible changes in embryonic structure happen throughout gastrulation as the cells that make up the different germ layers migrate and cause the previously round embryo to fold or invaginate into a cup-like appearance.<ref name=":1" />

Past gastrulation, an embryo continues to develop into a mature multicellular organism by forming structures necessary for life outside of the womb or egg. As the name suggests, organogenesis is the stage of embryonic development when organs form. During organogenesis, molecular and cellular interactions prompt certain populations of cells from the different germ layers to differentiate into organ-specific cell types.<ref>{{Cite web|url=https://embryo.asu.edu/pages/process-eukaryotic-embryonic-development|title=Process of Eukaryotic Embryonic Development {{!}} The Embryo Project Encyclopedia|website=embryo.asu.edu|access-date=2019-11-07|archive-date=2022-08-03|archive-url=https://web.archive.org/web/20220803034656/https://embryo.asu.edu/pages/process-eukaryotic-embryonic-development|url-status=live}}</ref> For example, in neurogenesis, a subpopulation of cells from the ectoderm segregate from other cells and further specialize to become the brain, spinal cord, or peripheral nerves.<ref>{{Cite journal|last1=Hartenstein|first1=Volker|last2=Stollewerk|first2=Angelika|date=2015-02-23|title=The Evolution of Early Neurogenesis|journal=Developmental Cell|volume=32|issue=4|pages=390–407|doi=10.1016/j.devcel.2015.02.004|pmid=25710527|pmc=5987553|issn=1534-5807}}</ref>

The embryonic period varies from species to species. In human development, the term fetus is used instead of embryo after the ninth week after conception,<ref>{{Cite web|url=https://www.medicinenet.com/embryo_vs_fetus_differences_week-by-week/article.htm|title=Embryo vs. Fetus: The First 27 Weeks of Pregnancy|website=MedicineNet|language=en|access-date=2019-11-07|archive-date=2022-07-23|archive-url=https://web.archive.org/web/20220723175538/https://www.medicinenet.com/embryo_vs_fetus_differences_week-by-week/article.htm|url-status=live}}</ref> whereas in [[zebrafish]], embryonic development is considered finished when a bone called the [[cleithrum]] becomes visible.<ref>{{Cite journal|last1=Kimmel|first1=Charles B.|last2=Ballard|first2=William W.|last3=Kimmel|first3=Seth R.|last4=Ullmann|first4=Bonnie|last5=Schilling|first5=Thomas F.|s2cid=19327966|date=1995|title=Stages of embryonic development of the zebrafish|journal=Developmental Dynamics|language=en|volume=203|issue=3|pages=253–310|doi=10.1002/aja.1002030302|pmid=8589427|issn=1097-0177|doi-access=free}}</ref> In animals that hatch from an egg, such as birds, a young animal is typically no longer referred to as an embryo once it has hatched. In [[Viviparity|viviparous]] animals (animals whose offspring spend at least some time developing within a parent's body), the offspring is typically referred to as an embryo while inside of the parent, and is no longer considered an embryo after birth or exit from the parent. However, the extent of development and growth accomplished while inside of an egg or parent varies significantly from species to species, so much so that the processes that take place after hatching or birth in one species may take place well before those events in another. Therefore, according to one textbook, it is common for scientists to interpret the scope of [[embryology]] broadly as the study of the development of animals.<ref name=":1" />