Editing Embryology (section)

===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>