View source for Embryology

{{short description|Branch of biology studying prenatal biology}}
{{about|the development of embryos in animals|the development of plant embryos|Sporophyte}}
{{Use dmy dates|date=September 2020}}
[[File:Blastulation.png|thumb|240px|right|'''1''' - morula, '''2''' - blastula]]
[[File:Gastrulation.png|thumb|240px|'''1''' - blastula, '''2''' - gastrula with blastopore; '''orange''' - ectoderm, '''red''' - endoderm]]
'''Embryology''' (from [[Ancient Greek|Greek]] ἔμβρυον, ''embryon'', "the unborn, [[embryo]]"; and -λογία, ''[[-logy|-logia]]'') is the branch of [[animal]] [[biology]] that studies the [[Prenatal development (biology)|prenatal development]] of [[gametes]] (sex cells), [[fertilization]], and development of [[embryo]]s and [[fetus]]es. Additionally, embryology encompasses the study of [[congenital]] disorders that occur before birth, known as [[teratology]].

Early embryology was proposed by [[Marcello Malpighi]], and known as [[preformationism]], the theory that organisms develop from pre-existing miniature versions of themselves. [[Aristotle]] proposed the theory that is now accepted, [[Epigenesis (biology)|epigenesis]]. [[Epigenesis (biology)|Epigenesis]] is the idea that organisms develop from seed or egg in a sequence of steps. Modern embryology developed from the work of [[Karl Ernst von Baer]], though accurate observations had been made in Italy by anatomists such as [[Aldrovandi]] and [[Leonardo da Vinci]] in the Renaissance.{{cn|date=November 2023}}

== Comparative embryology ==

=== Preformationism and epigenesis ===
[[File:Preformation.GIF|thumb|upright|A tiny person (a ''homunculus'') inside a [[sperm]], as drawn by [[Nicolaas Hartsoeker]] in 1695]]As recently as the 18th century, the prevailing notion in western human embryology was [[preformation]]: the idea that semen contains an embryo – a preformed, miniature infant, or ''homunculus'' – that simply becomes larger during development.

The competing explanation of embryonic development was ''[[Epigenesis (biology)|epigenesis]]'', originally proposed 2,000 years earlier by [[Aristotle]]. Much early embryology came from the work of the [[Italy|Italian]] anatomists [[Ulisse Aldrovandi|Aldrovandi]], [[Aranzio]], [[Leonardo da Vinci]], [[Marcello Malpighi]], [[Gabriele Falloppio]], [[Girolamo Cardano]], [[Emilio Parisano]], [[Fortunio Liceti]], [[Stefano Lorenzini]], [[Spallanzani]], [[Enrico Sertoli]], and [[Mauro Ruscóni]]. According to epigenesis, the form of an animal emerges gradually from a relatively formless egg. As [[microscopy]] improved during the 19th century, biologists could see that embryos took shape in a series of progressive steps, and epigenesis displaced preformation as the favored explanation among embryologists.

==== Cleavage ====
[[Cleavage (embryo)|Cleavage]] is the very beginning steps of a developing embryo. Cleavage refers to the many mitotic divisions that occur after the [[Egg cell|egg]] is fertilized by the [[sperm]]. The ways in which the cells divide is specific to certain types of animals and may have many forms.

==== Holoblastic ====
[[Holoblastic|Holoblastic cleavage]] is the complete division of cells. Holoblastic cleavage can be radial (see: [[Radial cleavage]]), spiral (see: [[Spiral cleavage]]), bilateral (see: [[Bilateral cleavage]]), or rotational (see: [[Rotational cleavage]]). In holoblastic cleavage, the entire egg will divide and become the embryo, whereas in meroblastic cleavage, some cells will become the embryo and others will be the yolk sac.

==== Meroblastic ====
[[Meroblastic|Meroblastic cleavage]] is the incomplete division of cells. The division furrow does not protrude into the yolky region as those cells impede membrane formation and this causes the incomplete separation of cells. Meroblastic cleavage can be bilateral (see: [[Bilateral cleavage]]), discoidal (see: [[Discoidal cleavage]]), or centrolecithal (see: [[Centrolecithal]]).

=== Basal phyla ===
Animals that belong to the basal phyla have holoblastic radial cleavage which results in radial symmetry (see: [[Symmetry in biology]]). During cleavage, there is a central axis that all divisions rotate about. The basal phyla also has only one to two embryonic cell layers, compared to the three in bilateral animals ([[endoderm]], [[mesoderm]], and [[ectoderm]]).

=== Bilaterians ===
In [[Bilateria|bilateral animals]], cleavage can be either holoblastic or meroblastic depending on the species. During [[gastrulation]], the [[blastula]] develops in one of two ways that divide the whole animal kingdom into two-halves (see: ''[[Embryological origins of the mouth and anus]]''). If in the [[blastula]], the first pore, or [[blastopore]], becomes the mouth of the animal, it is a [[protostome]]; if the blastopore becomes the anus, then it is a [[deuterostome]]. The [[protostomes]] include most [[invertebrate]] animals, such as insects, worms and molluscs, while the [[deuterostome]]s include a few invertebrates such as the [[echinoderms]] (starfish and relatives) and all the [[vertebrates]]. In due course, the [[blastula]] changes into a more differentiated structure called the [[gastrula]]. Soon after the [[gastrula]] is formed, three distinct layers of cells (the [[germ layers]]) from which all the bodily organs and tissues then develop.

==== Germ layers ====
* The innermost layer, or [[endoderm]], give rise to the digestive organs, the gills, lungs or swim bladder if present, and kidneys or nephrites.
* The middle layer, or [[mesoderm]], gives rise to the muscles, skeleton if any, and blood system.
* The outer layer of cells, or [[ectoderm]], gives rise to the nervous system, including the brain, and skin or carapace and hair, bristles, or scales.

====''Drosophila melanogaster'' (fruit fly)====
Drosophila have been used as a developmental model for many years. The studies that have been conducted have discovered many useful aspects of development that not only apply to fruit flies but other species as well.

Outlined below is the process that leads to cell and tissue differentiation.

# [[Maternal effect#Maternal effects in genetics|Maternal-effect genes]] help to define the anterior-posterior axis using [[Bicoid (gene)]] and [[Nanos (gene)]].
# [[Gap gene]]s establish 3 broad segments of the embryo.
# [[Pair-rule gene]]s define 7 segments of the embryo within the confines of the second broad segment that was defined by the gap genes.
# [[Segment-polarity gene]]s define another 7 segments by dividing each of the pre-existing 7 segments into anterior and posterior halves using a gradient of [[Hedgehog signaling pathway|Hedgehog]] and [[Wnt signaling pathway|Wnt]].
# [[Hox gene|Homeotic (Hox) genes]] use the 14 segments as pinpoints for specific types of cell differentiation and the histological developments that correspond to each cell type.

====Humans====
{{Main|Human embryonic development}}
Humans are [[Bilateria|bilateral animals]] that have holoblastic rotational cleavage. Humans are also [[deuterostome]]s. In regard to humans, the term embryo refers to the ball of dividing cells from the moment the [[zygote]] implants itself in the [[uterus]] wall until the end of the eighth week after conception. Beyond the eighth week after conception (tenth week of pregnancy), the developing human is then called a fetus.

== Evolutionary embryology ==
{{further|Evolutionary developmental biology}}

Evolutionary embryology is the expansion of comparative embryology by the ideas of [[Charles Darwin]]. Similarly to [[Karl Ernst von Baer]]'s principles that explained why many species often appear similar to one another in early developmental stages, Darwin argued that the relationship between groups can be determined based upon common embryonic and larval structures.

=== Von Baer's principles ===

# The general features appear earlier in development than do the specialized features.
# More specialized characters develop from the more general ones.
# The embryo of a given species never resembles the adult form of a lower one.
# The embryo of a given species does resemble the embryonic form of a lower one.<ref name=":0">{{Cite book|last1=Gilbert |first1=Scott F. |title=Developmental biology|last2=Barresi |first2=Michael J. F. |date=15 June 2016|isbn=978-1-60535-470-5|edition=Eleventh|location=Sunderland, Massachusetts|oclc=945169933}}</ref>

Using Darwin's theory evolutionary embryologists have since been able to distinguish between homologous and analogous structures between varying species. [[Homologous structure]]s are those that the similarities between them are derived from a common ancestor, such as the human arm and bat wings. [[Analogous Structure|Analogous structures]] are those that appear to be similar but have no common ancestral derivation.<ref name=":0" />

== Origins of modern embryology ==
Until the birth of modern embryology through observation of the mammalian ovum by [[Karl Ernst von Baer]] in 1827, there was no clear scientific understanding of embryology, although later discussions in this article show that some cultures had a fairly refined understanding of some of the principles. Only in the late 1950s when ultrasound was first used for uterine scanning, was the true developmental chronology of human fetus available. [[Karl Ernst von Baer]] along with [[Heinz Christian Pander]], also proposed the [[germ layer]] theory of development which helped to explain how the embryo developed in progressive steps. Part of this explanation explored why embryos in many species often appear similar to one another in early developmental stages using his four principles.

=== Modern embryology research ===
Embryology is central to [[evolutionary developmental biology]] ("evo-devo"), which studies the genetic control of the development process (e.g. [[morphogen]]s), its link to [[cell signalling]], its roles in certain diseases and [[mutation]]s, and its links to [[stem cell]] research. Embryology is the key to [[Gestational Surrogacy]], which is when the sperm of the intended father and egg of intended mother are fused in a lab forming an embryo. This embryo is then put into the surrogate who carries the child to term.

== Medical embryology ==
Medical embryology is used widely to detect abnormalities before birth. 2-5% of babies are born with an observable abnormality and medical embryology explores the different ways and stages that these abnormalities appear in.<ref name=":0" /> Genetically derived abnormalities are referred to as [[malformation]]s. When there are multiple malformations, this is considered a [[syndrome]]. When abnormalities appear due to outside contributors, these are disruptions. The outside contributors causing disruptions are known as [[teratogen]]s. Common teratogens are alcohol, retinoic acid,<ref>{{Cite journal|last1=Soprano|first1=Dianne Robert|last2=Soprano|first2=Kenneth J.|date=1995-07-01|title=Retinoids as Teratogens|journal=Annual Review of Nutrition|volume=15|issue=1|pages=111–132|doi=10.1146/annurev.nu.15.070195.000551|pmid=8527214|issn=0199-9885}}</ref> ionizing radiation or hyperthermic stress.

== Vertebrate and invertebrate embryology ==
Many principles of embryology apply to invertebrates as well as to vertebrates. Therefore, the study of invertebrate embryology has advanced the study of vertebrate embryology. However, there are many differences as well. For example, numerous invertebrate species release a [[larva]] before development is complete; at the end of the larval period, an animal for the first time comes to resemble an adult similar to its parent or parents. Although invertebrate embryology is similar in some ways for different invertebrate animals, there are also countless variations. For instance, while spiders proceed directly from egg to adult form, many insects develop through at least one larval stage.
For decades, a number of so-called normal staging tables were produced for the embryology of particular species, mainly focussing on external developmental characters. As variation in developmental progress makes comparison among species difficult, a character-based [[Standard Event System]] was developed, which documents these differences and allows for phylogenetic comparisons among species.<ref>{{cite journal |last1=Werneburg |first1=Ingmar |title=A Standard System to Study Vertebrate Embryos |journal=PLOS ONE |date=2009 |volume=4 |issue=6 |pages=e5887 |doi=10.1371/journal.pone.0005887|pmid=19521537 |pmc=2693928 |bibcode=2009PLoSO...4.5887W |doi-access=free }}</ref>

== Birth of developmental biology ==
After the 1950s, with the [[DNA]] helical structure being unraveled and the increasing knowledge in the field of [[molecular biology]], [[developmental biology]] emerged as a field of study which attempts to correlate the genes with morphological change, and so tries to determine which genes are responsible for each morphological change that takes place in an embryo, and how these genes are regulated.
<gallery>
File:Leonardo da Vinci - Studies of the foetus in the womb.jpg|Human [[embryo]]s by [[Leonardo da Vinci]]
File:6 weeks pregnant.png|Human embryo at six weeks gestational age
File:10dayMouseEmb.jpg|Histological film 10-day mouse embryo
</gallery>

As of today, human embryology is taught as a cornerstone subject in [[medical schools]], as well as in biology and zoology programs at both an undergraduate and graduate level.

==History==
{{Split section|History of embryology|discuss=Talk:Embryology#Splitting proposal |date=March 2025}}
===Ancient Egypt===
Knowledge of the placenta goes back at least to ancient Egypt, where it was viewed as the seat of the soul. There was an Egyptian official with the title ''Opener of the Kings Placenta''. An Egyptian text from the time of [[Akhenaten]] said that a human originates from the egg that grows in women.<ref>Joseph Needham, ''A History of Embryology'', Cambridge 1959,pp. 19-25.</ref>

===Ancient Asia===
Various interpretations of embryology have existed in Asia throughout history.<ref>Andreeva & Streavu (eds.), ''Transforming the Void: Embryological Discourse and Reproductive Imagery in East Asian Religions'', Brill 2015.</ref>  Included in the ancient Indian tradition of [[Ayurveda]] is ''garbhasharir'' or the study of embryology, which refers to conceptions of embryology from antiquity.<ref>{{Cite book |last=Needham |first=Joseph |url=https://catalog.hathitrust.org/Record/001503673 |title=A history of embryology. |date=1959 |publisher=Abelard-Schuman |location=New York}}</ref><ref>{{Cite journal |last=Khedikar |first=Sachin |date=April 14, 2016 |title=Critical Appraisal of Embryological Concepts (Garbhasharir) Delineated in Ayurveda |url=https://www.researchgate.net/publication/308524759 |journal=TM Journal |pages=45}}</ref> Descriptions of the [[amniotic sac]] appear in the ''[[Bhagavad Gita]], [[Bhagavata Purana]]'',<ref>The translation of one of the relevant texts may be accessed [https://vedabase.io/en/library/sb/3/31/ here].</ref> and the ''[[Sushruta Samhita]]''. One of the [[Upanishads]] known as the ''Garbhopanisaḍ'' states that the embryo is "like water in the first night, in seven nights it is like a bubble, at the end of half a month it becomes a ball. At the end of a month it is hardened, in two months the head is formed".<ref name=":1">John Wallingford, "Aristotle, Buddhist scripture and embryology in ancient Mexico: building inclusion by re-thinking what counts as the history of developmental biology", Development 2021.</ref> In Indian literature, the start of consciousness in an embryo is not clearly defined. Some scriptures state that it is active at conception, while others suggest that consciousness begins in the seventh to ninth month of fetal development. Many South Asian traditions, including some Tibetan traditions, believe that the fetus has conscious experiences towards the end of its development.<ref>{{Citation |last=Garrett |first=Frances Mary |title=Narratives of embryology: Becoming human in Tibetan literature. |date=2004-12-12 |work= |pages= |url=https://www.proquest.com/openview/95e3b64598c54d97202cdbf58cfb49f4/1?pq-origsite=gscholar&cbl=18750&diss=y |access-date= |publisher=University of Virginia |isbn=978-0-19-538004-0}}</ref>     

The development of the human embryo is mentioned in the ancient [[Buddhist]] text of Garbhāvakrāntisūtra (1st-4th century CE). It mentions the human gestation period of 38 days. The text describes embryonic development in first three weeks as a liquid part of yogurt and the differentiation of body parts such as arms, leg, feet and head in the third month.<ref>{{Cite web|last=Kritzer |first=Robert |title=Tibetan Texts of Garbhāvakrāntisūtra: Differences and Borrowings |url=https://www.academia.edu/6315789}}</ref><ref name=":1" />

===Ancient Greece===
====Pre-Socratic philosophers====
Many [[Pre-Socratic philosophy|pre-Socratic philosophers]] are recorded as having opinions on different aspects of embryology, although there is some bias in the description of their views in later authors such as [[Aristotle]]. According to [[Empedocles]] (whose views are described by [[Plutarch]] in the 1st century AD), who lived in the 5th century BC, the embryo derives and receives its blood from four vessels in all; two arteries and two veins. He also held sinews as originating from equal mixtures of earth and air. He further said men begin to form within the first month and are finished within fifty days. Asclepiades agreed that men are formed within fifty days, but he believed that women took a full two months to be fully knit. One observation, variously attributed to either [[Anaxagoras of Clazomenae]] or [[Alcmaeon of Croton]], says that the milk produced by mammals is analogous to the white of fowl egg. [[Diogenes of Apollonia]] said that a mass of flesh forms first, only then followed by the development of bone and nerves. Diogenes recognized that the placenta was a nutritional source for the growing fetus. He also said that the development of males took four months, but that the development of females took five months. He did not think the embryo was alive. Alcmaeon also made some contributions, and he is the first person reported to have practiced dissection. One idea, first stated by [[Parmenides]], was that there was a connection between the right side of the body and the male embryo, and between the left side of the body and the female embryo. According to [[Democritus]] and [[Epicurus]], the fetus is nourished at the mouth inside the mother and there are comparable teats that supply this nourishment within the mother's body to the fetus.<ref>Joseph Needham, ''A History of Embryology'', Cambridge 1959, pp. 27-31.</ref> Discussion on various views regarding how long it takes for specific parts of the embryo to form appear in an anonymous document known as the ''Nutriment''.

Ancient Greeks discussed whether only the male had a seed which developed into the embryo within the female womb, or both the male and the female each had a seed that made a contribution to the developing embryo. The difficulty that one-seed theorists confronted was to explain the maternal resemblance of the progeny. One issue that two-seed theorists confronted was why the female seed was needed if the male already had a seed. One common solution to this problem was to assert that the female seed was either inferior or inactive. Another question was the origin of the seed. The encephalomyelogenic theory stated that the seed originated from the brain or and/or bone marrow. Later came pangenesis, which asserted the seed was drawn from the whole body in order to explain the general resemblance in the body of the offspring. Later on hematogenous theory developed which asserted that the seed was drawn from the blood. A third question was how or in what form the progeny existed in the seed prior to developing into an embryo and a fetus. According to preformationists, the body of the progeny already existed in a pre-existing but undeveloped form in the seed. Three variants of preformationism were homoiomerous preformationism, anhomoiomerous preformationism, and homuncular preformationism. According to the first, the homoiomerous parts of the body (e.g. humors, bone) already exist pre-formed in the seed. The second held that it was the anhomoiomerous parts that were pre-formed. Finally, the third view held that the whole was already a unified organic thing. Preformationism was not the only view. According to epigenesists, parts of the embryo successively form after conception takes place.<ref name="wilb">James Wilberding, "Plato's Embryology," Early Science and Medicine 2015.</ref>

====Hippocrates====
Some of the most well-known early ideas on embryology come from [[Hippocrates]] and the [[Hippocratic Corpus]], where discussion on the embryo is usually given in the context of discussing [[obstetrics]] (pregnancy and childbirth). Some of the most relevant Hippocratic texts on embryology include the ''Regimen on Acute Diseases'', ''On Semen'', and ''On the Development of the Child''. Hippocrates claimed that the development of the embryo is put into motion by fire and that nourishment comes from food and breath introduced into the mother. An outer layer of the embryo solidifies, and the fire within consumes humidity which makes way for development of bone and nerve. The fire in the innermost part becomes the belly and air channels are developed in order to route nourishment to it. The enclosed fire also helps form veins and allows for circulation. In this description, Hippocrates aims at describing the causes of development rather than describing what develops. Hippocrates also develops views similar to [[preformationism]], where he claims that all parts of the embryo simultaneously develop. Hippocrates also believed that maternal blood nourishes the embryo. This blood flows and coagulates to help form the flesh of the fetus. This idea was derived from the observation that menstrual blood ceases during pregnancy, which Hippocrates took to imply that it was being redirected to fetal development. Hippocrates also claimed that the flesh differentiates into different organs of the body, and Hippocrates saw as analogous an experiment where  a mixture of substances placed into water will differentiate into different layers. Comparing the seed to the embryo, Hippocrates further compared the stalk to the umbilical cord.<ref>Joseph Needham, ''A History of Embryology'', Cambridge 1959, pp. 31-37.</ref>

====Aristotle====

Some embryological discussion appears in the writings of Aristotle's predecessor [[Plato]], especially in his ''[[Timaeus (dialogue)|Timaeus]]''. One of his views were that the bone marrow acted as the seedbed, and that the soul itself was the seed out of which the embryo developed, though he did not explain how this development proceeded. Scholars also continue to debate the views he held on various other aspects of embryology.<ref name="wilb" /> However, a much more voluminous discussion on the subject comes from the writings of [[Aristotle]], especially as appears in his ''[[On the Generation of Animals]]''.<ref>Cera Lawrence. "On the Generation of Animals, by Aristotle". The Embryo Project Encyclopedia. 2010. [https://embryo.asu.edu/pages/generation-animals-aristotle Accessible].</ref> Some ideas related to embryology also appear in his ''[[History of Animals]]'', ''[[On the Parts of Animals]]'', ''On Respiration'', and ''On the Motion of Animals''. Means by which we know Aristotle studied embryology, and most likely his predecessors as well, was through studying developing embryos taken out from animals as well as aborted and miscarried human embryos. Aristotle believed that the female supplied the matter for the development of the embryo, formed from the menstrual blood whereas the semen that comes from the male shapes that matter. Aristotle's belief that both the male and female made a contribution to the actual fetus goes against some prior beliefs. According to [[Aeschylus]] and some Egyptian traditions, the fetus solely develops from the male contribution and that the female womb simply nourishes this growing fetus. On the other hand, the [[Melanesians]] held that the fetus is solely a product of the female contribution. Aristotle did not believe there were any external influences on the development of the embryo. Against Hippocrates, Aristotle believed that new parts of the body developed over time rather than all forming immediately and developing from then on. He also considered whether each new part derives from a previously formed part or develops independently of any previously formed part. On the basis that different parts of the body do not resemble each other, he decided in favor of the latter view. He also described development of fetal parts in terms of mechanical and automatic processes. In terms of the development of the embryo, he says it begins in a liquid-like state as the material secreted by the female combines with the semen of the male, and then the surface begins to solidify as it interacts with processes of heating and cooling. The first part of the body to differentiate is the heart, which Aristotle and many of his contemporaries believed was the location of reason and thinking. Aristotle claimed that vessels join to the uterus in order to supply nourishment to the developing fetus. Some of the most solid parts of the fetus cool and, as they lose moisture to heat, turn into nails, horns, hoofs, beaks, etc. Internal heat dries away moisture and forms sinews and bones and the skin results from drying of the flesh. Aristotle also describes the development of birds in eggs at length. He further described embryonic development in dolphins, some sharks, and many other animals. Aristotle singularly wrote more on embryology than any other pre-modern author, and his influence on the subsequent discussion on the subject for many centuries was immense, introducing into the subject forms of classification, a comparative method from various animals, discussion of the development of sexual characteristics, compared the development of the embryo to mechanistic processes, and so forth.<ref>Joseph Needham, ''A History of Embryology'', Cambridge 1959, pp. 37-60.</ref>

====Later Greek embryology====
Reportedly, some [[Stoicism|Stoics]] claimed that most parts of the body formed at once during embryological development. Some [[Epicureanism|Epicureans]] claimed that the fetus is nourished by either the amniotic fluid or the blood, and that both male and female supply material to the development of the fetus. According to the writings of [[Tertullian]], [[Herophilus]] in the 5th century BC described the ovaries and fallopian tubes (but not past what was already described by Aristotle) and also dissected some embryos. One advance Herophilus made, against the conceptions of other individuals such as Aristotle, was that the brain was the center of intellect rather than the heart. Though not a part of Greek tradition, in [[Book of Job|Job]] 10, the formation of the embryo is likened to the curdling of milk into cheese, as described by Aristotle. Whereas Needham sees this statement in Job as part of the Aristotelian tradition, others see it as evidence that the milk analogy predates the Aristotelian Greek tradition and originates in Jewish circles.<ref name="kot">Samuel Kottek, "Embryology in Talmudic and Midrashic Literature", Journal of the History of Biology 1981.</ref> In addition, the [[Wisdom of Solomon]] (7:2) also has the embryo formed from menstrual blood. [[Soranus of Ephesus]] also wrote texts on embryology which went into use for a long time. Some rabbinic texts discuss the embryology of a female Greek writer named Cleopatra, a contemporary of [[Galen]] and Soranus, who was said to have claimed that the male fetus is complete in 41 days whereas the female fetus is complete in 81 days. Various other texts of less importance also appear and describe various aspects of embryology, though without making much progress from Aristotle. [[Plutarch]] has a chapter in one of his works titled "Whether was before, the hen or egg?" Discussion on embryological tradition also appears in many [[Neoplatonism|Neoplatonic traditions]].<ref>James Wilberding, ''Forms, Souls, and Embryos: Neoplatonists on Human Reproduction'', Routledge 2017.</ref>

Next to Aristotle, the most impactful and important Greek writer on biology was Galen of Pergamum, and his works were transmitted throughout the [[Middle Ages]]. Galen discusses his understanding of embryology in two of his texts, those being his ''On the Natural Faculties'' and his ''On the Formation of the Foetus''.<ref>Michael Boylan, ''Galen's Conception Theory'', Journal  of the History of Biology 1986.</ref> There is an additional text spuriously attributed to Galen known as ''On the Question of whether the Embryo is an Animal''. Galen described embryological development in four stages. In the first stage, the semen predominates. In the second stage, the embryo is filled with blood. In the third stage, the main outlines of the organs have developed but various other parts remain undeveloped. In the fourth stage, formation is complete and has reached a stage where we can call it a child. Galen described processes that played a role in furthering development of the embryo such as warming, drying, cooling, and combinations thereof. As this development plays out, the form of life of the embryo also moves from that like a plant to that of an animal (where the analogy between the root and umbilical cord is made). Galen claimed that the embryo forms from menstrual blood, by which his experimental analogy was that when you cut the vein of an animal and allow blood to flow out and into some mildly heated water, a sort of coagulation can be observed. He gave detailed descriptions of the position of the umbilical cord relative to other veins.<ref>Joseph Needham, ''A History of Embryology'', Cambridge 1959, pp. 60-74.</ref>

===Patristics===

The question of embryology is discussed among a number of [[List of early Christian writers|early Christian writers]], largely in terms of theological questions such as whether the fetus has value and/or when it begins to have value. (Although a number of Christian authors continued the classical discussions on the description of the development of the embryo, such as [[Jacob of Serugh]].<ref name="kour">Yousef Kouriyhe. "Jakob von Sarug (451-521): Brief an den Erzdiakon Mar Julian — Edessa — 451-521 (Syrisch) — Mekka II — TUK_0955". Corpus Coranicum. [https://corpuscoranicum.de/kontexte/index/sure/23/vers/14 Available].</ref> Passing reference to the embryo also appears in the eighth hymn of [[Ephrem the Syrian]]'s ''Paradise Hymns''.<ref>Sebastian Brock (translator), ''Saint Ephrem: Hymns on Paradise'', St. Vladimir's Seminary Press 1990, p133.</ref>) Many [[patristic]] treatments of embryology continued in the stream of Greek tradition.<ref>Peter Kitzler, "Tertullian and Ancient Embryology in De carne Christi 4,1 and 19,3−4," Journal of Ancient Christianity 2014.</ref> The earlier Greek and Roman view that it was not was reversed and all pre-natal infanticide was condemned. [[Tertullian]] held that the soul was present from the moment of conception. The [[Quinisext Council]] concluded that "we pay no attention to the subtle division as to whether the foetus is formed or unformed". In this time, then, the Roman practice of child exposure came to an end, where unwanted yet birthed children, usually females, were discarded by the parents to die.<ref>W.V. Harris, "Child Exposure in the Roman Empire," Journal of Roman Studies 1994.</ref> Other more liberal traditions followed [[Augustine]], who instead viewed that the animation of life began on the 40th day in males and the 80th day in females but not prior. Before the 40th day for men and 80th day for women, the embryo was referred to as the ''embryo informatus'', and after this period was reached, it was referred to as the ''embryo formatus''. The notion originating from the Greeks that the male embryo developed faster remained in various authors until it was experimentally disproven by Andreas Ottomar Goelicke in 1723.<ref>Joseph Needham, ''A History of Embryology'', Cambridge 1959, pp. 75-77.</ref>

Various patristic literature from backgrounds ranging from [[Nestorian]], [[Miaphysite]] and [[Chalcedonian]] discuss and choose between three different conceptions on the relation between the soul and the embryo. According to one view, the soul pre-exists and enters the embryo at the moment of conception (''prohyparxis''). According to a second view, the soul enters into existence at the moment of conception (''synhyparxis''). In a third view, the soul enters into the body after it has been formed (''methyparxis''). The first option was proposed by [[Origen]], but was increasingly rejected after the fourth century. On the other hand, the other two options were equally accepted after this point. The second position appears to have been proposed as a response to Origen's notion of a pre-existing soul. After the sixth century, the second position was also increasingly seen as Origenist and so rejected on those grounds. The writings of Origen were condemned during the Second [[Origenist Crises]] in 553. Those defending ''prohyparxis'' usually appealed to the Platonic notion of an eternally moving soul. Those defending the second position also appealed to Plato but rejected his notion on the eternality of the soul. Finally, those appealing to the third position appealed both to Aristotle and scripture. Aristotelian notions included the progression of the development of the soul, from an initial plant-like soul, to a sensitive soul found in animals and allows for movement and perception, and finally the formation of a rational soul which can only be found in the fully-formed human. Furthermore, some scriptural texts were seen as implying the formation of the soul temporally after the formation of the body (namely Genesis 2:7; Exodus 21:22-23; Zachariah 12:1). In the ''De hominis opificio'' of [[Gregory of Nyssa]], Aristotle's triparitate notion of the soul was accepted. Gregory also held that the rational soul was present at conception. [[Theodoret]] argued based on Genesis 2:7 and Exodus 21:22 that the embryo is only ensouled after the body is fully formed. Based on Exodus 21:22 and Zachariah 12:1, [[Philoxenus of Mabbug]] claimed that the soul was created in the body forty days after conception. In his ''De opificio mundi'', the Christian philosopher [[John Philoponus]] claimed that the soul is formed after the body. Later still, the author [[Leontius]] held that the body and soul were created simultaneously, though it is also possible he held that the soul pre-existed the body.<ref name="dirk">Dirk Krausmuller, "When Christology intersects with embryology: the viewpoints of Nestorian, Monophysite and Chalcedonian authors of the sixth to tenth centuries", Byzantinische Zeitschrift 2020.</ref>

Some Miaphysites and Chalcedonians seemed to have been compelled into accepting ''synhyparxis'' in the case of Jesus because of their view that the incarnation of Christ resulted in both one hypostasis and one nature, whereas some Nestorians claimed that Christ, like us, must have had his soul formed after the formation of his body because, per Hebrews 4:15, Christ was like us in all ways but sin. (On the other hand, Leontinus dismissed the relevance of Hebrews 4:15 on the basis that Christ differed from us not only in sinfulness but also conception without semen, making ''synhyparxis'' another of Christ's supernatural feats.) They felt comfortable holding this view, under their belief that the human nature of Jesus was separate from the divine hypostasis. Some Nestorians still wondered, however, if the body united with the soul in the moment the soul was created or whether it came with it only later. The Syriac author [[Babai the Great|Babai]] argued for the former on the basis that the latter was hardly better than [[adoptionism]]. [[Maximus the Confessor]] ridiculed the Aristotelian notion of the development of the soul on the basis that it would make humans parents of both plants and animals. He held to ''synhyparxis'' and regarded the other two positions both as incorrect extremes. After the 7th century, Chalcedonian discussion on embryology is slight and the few works that touch on the topic support ''synhyparxis''. But debate among other groups remains lively, still divided on similar sectarian grounds. The patriarch [[Timothy I of Seleucia-Ctesiphon|Timothy I]] argued that the Word first united with the body, and only later with the soul. He cited John 1:1, claiming on its basis that the Word became flesh first, not a human being first. Then, [[Jacob of Edessa]] rejected ''prohyparxis'' because Origen had defended it and ''methyparxis'' because he believed that it made the soul ontologically inferior and as only being made for the body. Then, [[Moses Bar Kepha]] claimed, for Christological reasons as a Miaphysite, that only ''synhyparxis'' was acceptable. He claimed that Genesis 2:7 has no temporal sequence and that Exodus 21:22 regards the formation of the body and not the soul and so is not relevant. To argue against ''methyparxis'', he reasoned that body and soul are both present at death and, because what is at the end must correspond to what is also at the beginning, conception must also have body and soul together.<ref name="dirk" />

===Embryology in Jewish tradition===

Many Jewish authors also discussed notions of embryology, especially as they appear in the [[Talmud]]. Much of the embryological data in the Talmud is part of discussions related to the impurity of the mother after childbirth. The embryo was described as the ''peri habbetten'' (fruit of the body) and it developed through various stages: (1) ''golem'' (formless and rolled-up) (2) ''shefir meruqqam'' (embroidered foetus) (3) ''ubbar'' (something carried) (4) ''walad'' (child) (5) ''walad shel qayama'' (viable child) (6) ''ben she-kallu khadashaw'' (child whose months have been completed). 

Some mystical notions regarding embryology appear in the [[Sefer Yetzirah]]. The text in the [[Book of Job]] relating to the fetus forming by analogy to the curdling of milk into cheese was cited in the Babylonian Talmud and in even greater detail in the [[Midrash]]: "When the womb of the woman is full of retained blood which then comes forth to the area of her menstruation, by the will of the Lord comes a drop of white-matter which falls into it: at once the embryo is created. [This can be] compared to milk being put in a vessel: if you add to it some lab-ferment [drug or herb], it coagulates and stands still; if not, the milk remains liquid."<ref name="kot" /> The Talmud sages held that there were two seeds that participated in the formation of the embryo, one from the male and one from the female, and that their relative proportions determine whether that develops into a male or a female. 

In the Tractate Nidda, the mother was said to provide a "red-seed" which allows for the development of skin, flesh, hair, and the black part of the eye (pupil), whereas the father provides the "white-seed" which forms the bones, nerves, brain, and the white part of the eye. And finally, God himself was thought to provide the spirit and soul, facial expressions, capacity for hearing and vision, movement, comprehension, and intelligence. Not all strands of Jewish tradition accepted that both the male and female contributed parts to the formation of the fetus. 

The 13th century medieval commentator [[Nachmanides]], for example, rejected the female contribution. In Tractate Hullin in the Talmud, whether the organs of the child resemble more closely those of the mother or father is said to depend on which one contribute more matter to the embryo depending on the child. Rabbi Ishmael and other sages are said to have disagreed on one matter: they agreed that the male embryo developed on the 41st day, but disagreed on whether this was the case for the female embryo. Some believed that the female embryo was complete later, whereas others held that they were finished at the same time. The only ancient Jewish authors who associated abortion with homicide were [[Josephus]] and [[Philo of Alexandria]] in the 1st century. In the Talmud, a child is granted humanness at birth, while other rabbinical texts place it at the 13th postnatal day.<ref>{{Cite journal |last=Schenker |first=Joseph G. |date=June 2008 |title=The beginning of human life |url=http://dx.doi.org/10.1007/s10815-008-9221-6 |journal=Journal of Assisted Reproduction and Genetics |volume=25 |issue=6 |pages=271–276 |doi=10.1007/s10815-008-9221-6 |pmid=18551364 |pmc=2582082 |issn=1058-0468}}</ref>   

Some Talmudic texts discuss magical influences on the development of the embryo, such as one text which claims that if one sleeps on a bed that is pointed to the north–south will have a male child. According to Nachmanides, a child born of a cold drop of semen will be foolish, one born from a warm drop of semen will be passionate and irascible, and one born from a semen drop of medium temperature will be clever and level-headed. Some Talmudic discussions follow from [[Hippocrates|Hippocratic]] claims that a child born on the eighth month could not survive, whereas others follow Aristotle in claiming that they sometimes could survive. One text even says that survival is possible on the seventh month, but not the eighth. Talmudic embryology, in various aspects, follows Greek discourses especially from Hippocrates and Aristotle, but in other areas, makes novel statements on the subject.<ref name="kot" />

Judaism allows assisted reproduction, such as IVF embryo transfer and maternal surrogacy, when the spermatozoon and oocyte originate from the respective husband and wife.<ref>{{Cite journal |last=Schenker |first=Joseph G. |date=2013-09-03 |title=Human reproduction: Jewish perspectives |url=http://dx.doi.org/10.3109/09513590.2013.825715 |journal=Gynecological Endocrinology |volume=29 |issue=11 |pages=945–948 |doi=10.3109/09513590.2013.825715 |pmid=24000935 |issn=0951-3590|url-access=subscription }}</ref> 

===Embryology in the Islamic tradition===

Passing reference to embryological notions also appear in the [[Qur'an]] (22:5), where the development of the embryo proceeds in four stages from drop, to a clinging clot, to a partially developed stage, to a fully developed child.<ref>Joseph Needham, ''A History of Embryology'', Cambridge 1959, pp. 74-82.</ref> The notion of clay turning into flesh is seen by some as analogous to a text by [[Theodoret]] that describes the same process.<ref>Emmanouela Grypeou. "Theodoret von Kyrrhos: Kompendium häretischer Erdichtungen V.9 — Syrien — ca. 450 n.Chr. (Griechisch) — Mekka II — TUK_1235". Corpus Coranicum. [https://corpuscoranicum.de/kontexte/index/sure/23/vers/12 Available].</ref> The four stages of development in the Qur'an are similar to the four stages of embryological development as described by [[Galen]]. In the early 6th century, [[Sergius of Reshaina]] devoted himself to the translation of Greek medical texts into Syriac and became the most important figure in this process. Included in his translations were the relevant embryological texts of Galen. Anurshirvan founded a medical school in the southern Mesopotamian city of [[Gundeshapur]], known as the [[Academy of Gondishapur]], which also acted as a medium for the transmission, reception, and development of notions from Greek medicine. These factors helped the transmission of Greek notions on embryology, such as found in Galen, to enter into the Arabian milieu.<ref>Michael Marx. "Galen von Pergamon (129-199): Galen De Semine I, 8 — Kleinasien und Rom — 2. Jh. n.Chr. (Griechisch) — Mekka II — TUK_0986". Corpus Coranicum. [https://corpuscoranicum.de/kontexte/index/sure/23/vers/13 Available].</ref> Very similar embryonic descriptions also appear in the Syriac [[Jacob of Serugh]]'s letter to the Archdeacon Mar Julian.<ref name="kour" />

Embryological discussions also appear in the Islamic legal tradition.<ref>Ghaly, Mohammed. "Human Embryology in the Islamic Tradition The Jurists of the Post-formative Era in Focus," Islamic Law and Society (2014).</ref>

==See also==
{{col div|colwidth=30em}}
* [[Abortion]]
* [[Caspar Friedrich Wolff]]
* [[Embryogenesis]]
* [[Embryonic differentiation waves]]
* [[Embryology of digestive system and the body cavities]]
* [[French flag model]]
* [[Morphogen]]
* [[Ontogeny]]
* [[Recapitulation theory]]
{{colend}}

== References ==
=== Citations ===
{{Reflist}}

=== Sources ===
* "Germ layer." Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 6 Nov. 2009 <https://www.britannica.com/EBchecked/topic/230597/germ-layer>.
* Welton, John. "| Circle Surrogacy". Circle Surrogacy, Full Service, Worldwide Surrogate-Parenting Agency, www.circlesurrogacy.com/about/what-is-surrogacy.

==Further reading==
* {{cite book|last1=Apostoli|first1=Pietro|last2=Catalani|first2=Simona|editor=Astrid Sigel, Helmut Sigel and Roland K. O. Sigel|title=Metal Ions in Toxicology|series=Metal Ions in Life Sciences|volume=8|year=2011|publisher=RSC Publishing|doi=10.1039/9781849732116-00263|pages=263–303|chapter=Chapter 11. Metal Ions Affecting Reproduction and Development|isbn=978-1-84973-091-4}}
* Scott F. Gilbert. ''Developmental Biology''. Sinauer, 2003. {{ISBN|0-87893-258-5}}.
* Lewis Wolpert. ''Principles of Development''. Oxford University Press, 2006. {{ISBN|0-19-927536-X}}.
* {{cite book|last1=Carlson|first1=Bruce M.|last2=Kantaputra|first2=Piranit N.|title=Human embryology and developmental biology|date=2014|publisher=Elsevier/Saunders|location=Philadelphia, PA|isbn=978-1-4557-2794-0}} ([https://www.youtube.com/watch?v=snSp1Szvh6Y click here for more information])

==External links==
{{commons category|Embryology}}
{{EB1911 Poster|Embryology}}
* [http://www.embryology.ch/indexdu.html Online course in embryology] {{Webarchive|url=https://web.archive.org/web/20210224215746/http://www.embryology.ch/indexdu.html |date=24 February 2021 }}
* [http://www.indiana.edu/~anat550/embryo_main/index.html Indiana University's Human Embryology Animations] {{Webarchive|url=https://web.archive.org/web/20191016062333/http://www.indiana.edu/~anat550/embryo_main/index.html |date=16 October 2019 }}
* [http://embryology.med.unsw.edu.au UNSW Embryology] Large resource of information and media.
* [https://web.archive.org/web/20120225084041/http://www.cambridgenetwork.co.uk/views/biolines/ What is a human admixed embryo?]
* [http://www2.merriam-webster.com/cgi-bin/mwmednlm?book=Medical&va=embryo Definition of embryo according to Webster] {{Webarchive|url=https://web.archive.org/web/20100130235336/http://www2.merriam-webster.com/cgi-bin/mwmednlm?book=Medical&va=embryo |date=30 January 2010 }}

{{Embryology}}
{{Embryology of head and neck}}
{{Ossification}}
{{Development of circulatory system}}
{{Development of nervous system}}
{{Development of respiratory system}}
{{Development of digestive system}}
{{Development of urinary and reproductive systems}}
{{Developmental biology}}
{{Branches of biology}}
{{Authority control}}

[[Category:Embryology| ]]