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Embryology (from
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
ἔμβρυον, ''embryon'', "the unborn, embryo"; and -λογία, ''
-logia ''-logy'' is a suffix in the English language, used with words originally adapted from Ancient Greek ending in ('). The earliest English examples were anglicizations of the French '' -logie'', which was in turn inherited from the Latin '' -logi ...
'') is the branch of
animal Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...
that studies the prenatal development of
gametes A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...
(sex cells), fertilization, and development of embryos and
fetus A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal dev ...
es. Additionally, embryology encompasses the study of congenital disorders that occur before birth, known as teratology. Early embryology was proposed by
Marcello Malpighi Marcello Malpighi (10 March 1628 – 30 November 1694) was an Italian biologist and physician, who is referred to as the "Founder of microscopical anatomy, histology & Father of physiology and embryology". Malpighi's name is borne by several ph ...
, and known as
preformationism In the history of biology, preformationism (or preformism) is a formerly popular theory that organisms develop from miniature versions of themselves. Instead of assembly from parts, preformationists believed that the form of living things exist, ...
, the theory that organisms develop from pre-existing miniature versions of themselves.
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ph ...
proposed the theory that is now accepted, epigenesis. 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 Karl Ernst Ritter von Baer Edler von Huthorn ( – ) was a Baltic German scientist and explorer. Baer was a naturalist, biologist, geologist, meteorologist, geographer, and is considered a, or the, founding father of embryology. He was ...
, though accurate observations had been made in Italy by anatomists such as Aldrovandi and Leonardo da Vinci in the Renaissance.


Comparative embryology


Preformationism and epigenesis

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'', originally proposed 2,000 years earlier by
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ph ...
. Much early embryology came from the work of the
Italian Italian(s) may refer to: * Anything of, from, or related to the people of Italy over the centuries ** Italians, an ethnic group or simply a citizen of the Italian Republic or Italian Kingdom ** Italian language, a Romance language *** Regional Ita ...
anatomists Aldrovandi, Aranzio, Leonardo da Vinci,
Marcello Malpighi Marcello Malpighi (10 March 1628 – 30 November 1694) was an Italian biologist and physician, who is referred to as the "Founder of microscopical anatomy, histology & Father of physiology and embryology". Malpighi's name is borne by several ph ...
,
Gabriele Falloppio Gabriele Falloppio (also Gabrielle Falloppia) (1522/23 – 9 October 1562) was an Italian anatomist often known by his Latin name Fallopius. He was one of the most important anatomists and physicians of the sixteenth century, giving his name ...
,
Girolamo Cardano Gerolamo Cardano (; also Girolamo or Geronimo; french: link=no, Jérôme Cardan; la, Hieronymus Cardanus; 24 September 1501– 21 September 1576) was an Italian polymath, whose interests and proficiencies ranged through those of mathematician, ...
,
Emilio Parisano Emilio may refer to: * Emilio Navaira, a Mexican-American singer often called "Emilio" * Emilio Piazza Memorial School, in Port Harcourt, Rivers State * Emilio (given name) * ''Emilio'' (film), a 2008 film by Kim Jorgensen See also * Emílio (dis ...
,
Fortunio Liceti Fortunio Liceti (Latin: ''Fortunius Licetus''; October 3, 1577 – May 17, 1657), was an Italian physician and philosopher. Life and career He was born prematurely at Rapallo, near Genoa to Giuseppe Liceti and Maria Fini, while the family was m ...
,
Stefano Lorenzini Stefano is the Italian form of the masculine given name Στέφανος (Stefanos, Stephen). The name is of Greek origin, Στέφανος, meaning a person who made a significant achievement and has been crowned. In Orthodox Christianity the ach ...
,
Spallanzani Lazzaro Spallanzani (; 12 January 1729 – 11 February 1799) was an Italian Catholic priest (for which he was nicknamed Abbé Spallanzani), biologist and physiologist who made important contributions to the experimental study of bodily function ...
,
Enrico Sertoli Enrico Sertoli (June 6, 1842, Sondrio – January 28, 1910, Sondrio) was an Italian physiologist, histologist, anatomist, biologic chemist, physician, teacher and inventor. He is remembered for his discovery regarding the branched cells of semin ...
, 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. CLEVAGE Cleavage is the very beginning steps of a developing embryo. Cleavage refers to the many mitotic divisions that occur after the 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 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 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 Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take the face of a human being which has a pla ...
). 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 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 deuterostomes include the
vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...
. In due course, the blastula changes into a more differentiated structure called the
gastrula Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals the blastocyst is reorganized into a multilayered structure known as the gastrula. ...
. Soon after the
gastrula Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals the blastocyst is reorganized into a multilayered structure known as 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 genes help to define the anterior-posterior axis using Bicoid (gene) and
Nanos (gene) ''Drosophila'' embryogenesis, the process by which ''Drosophila'' (fruit fly) embryos form, is a favorite model system for genetics and developmental biology. The study of its embryogenesis unlocked the century-long puzzle of how development w ...
. # Gap genes establish 3 broad segments of the embryo. # Pair-rule genes define 7 segments of the embryo within the confines of the second broad segment that was defined by the gap genes. # Segment-polarity genes define another 7 segments by dividing each of the pre-existing 7 segments into anterior and posterior halves using a gradient of
Hedgehog A hedgehog is a spiny mammal of the subfamily Erinaceinae, in the eulipotyphlan family Erinaceidae. There are seventeen species of hedgehog in five genera found throughout parts of Europe, Asia, and Africa, and in New Zealand by introductio ...
and Wnt. # 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

Humans are bilateral animals that have holoblastic rotational cleavage. Humans are also deuterostomes. In regard to humans, the term embryo refers to the ball of dividing cells from the moment the
zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicell ...
implants itself in the
uterus The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The ...
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

Evolutionary embryology is the expansion of comparative embryology by the ideas of
Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended ...
. Similarly to
Karl Ernst von Baer Karl Ernst Ritter von Baer Edler von Huthorn ( – ) was a Baltic German scientist and explorer. Baer was a naturalist, biologist, geologist, meteorologist, geographer, and is considered a, or the, founding father of embryology. He was ...
'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. Using Darwin's theory evolutionary embryologists have since been able to distinguish between homologous and analogous structures between varying species. Homologous structures are those that the similarities between them are derived from a common ancestor, such as the human arm and bat wings.
Analogous structures Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last co ...
are those that appear to be similar but have no common ancestral derivation.


Origins of modern embryology

Until the birth of modern embryology through observation of the mammalian ovum by
Karl Ernst von Baer Karl Ernst Ritter von Baer Edler von Huthorn ( – ) was a Baltic German scientist and explorer. Baer was a naturalist, biologist, geologist, meteorologist, geographer, and is considered a, or the, founding father of embryology. He was ...
in 1827, there was no clear scientific understanding of embryology. 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 Karl Ernst Ritter von Baer Edler von Huthorn ( – ) was a Baltic German scientist and explorer. Baer was a naturalist, biologist, geologist, meteorologist, geographer, and is considered a, or the, founding father of embryology. He was ...
along with
Heinz Christian Pander Heinz Christian Pander, also Christian Heinrich Pander ( – ), was a Russian Empire ethnic Baltic German biologist and embryologist. Biography In 1817 he received his doctorate from the University of Würzburg, and spent several years (1827� ...
, 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. morphogens), its link to cell signalling, its roles in certain diseases and mutations, and its links to stem cell research. Embryology is the key to
Gestational Surrogacy Surrogacy is an arrangement, often supported by a legal agreement, whereby a woman agrees to delivery/labour for another person or people, who will become the child's parent(s) after birth. People may seek a surrogacy arrangement when pregnan ...
, 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. Genetically derived abnormalities are referred to as malformations. When there are multiple malformations, this is considered a
syndrome A syndrome is a set of medical signs and symptoms which are correlated with each other and often associated with a particular disease or disorder. The word derives from the Greek language, Greek σύνδρομον, meaning "concurrence". When a sy ...
. When abnormalities appear due to outside contributors, these are disruptions. The outside contributors causing disruptions are known as teratogens. Common teratogens are alcohol, retinoic acid, 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 A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. ...
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 The ''"Standard Event System" (SES) to Study Vertebrate Embryos'' was developed in 2009 to establish a common language in comparative embryology.Werneburg (2009). A Standard System to Study Vertebrate Embryos. PLoS ONE 4(6): e5887. http://www.plos ...
was developed, which documents these differences and allows for phylogenetic comparisons among species.


Birth of developmental biology

After the 1950s, with the DNA helical structure being unraveled and the increasing knowledge in the field of
molecular biology Molecular biology is the branch of biology that seeks to understand the molecular basis of biological activity in and between cells, including biomolecular synthesis, modification, mechanisms, and interactions. The study of chemical and physi ...
, 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. File:Leonardo da Vinci - Studies of the foetus in the womb.jpg, Human embryos 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 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


Ancient Egypt

The study of embryology has a long pedigree. Knowledge of the placenta goes back at least to ancient Egypt, where the placenta was viewed as the seat of the soul. There was even an Egyptian official who held the title ''Opener of the Kings Placenta''. Furthermore, one Egyptian text from the time of Akhenaten claims that a human originates from the egg that grows in women.


Ancient India

A variety of conceptions on embryology appeared in ancient Asia. A more advanced understanding of the embryological process was known from ancient India. Descriptions of the amniotic sac appear in the '' Bhagavad Gita, Bhagavata Purana'', and the '' Sushruta Samhita''. For example, the Sushruta Samhita claims that an embryo emerges from semen and blood, both of which in turn find their origins in chyle. In the third month, differentiation of body parts such as arms, leg, and head occurs, and this is followed in the fourth month by the development of the heart, thorax, and abdomen. In the sixth month, the hair, bones, sinews, nails, and veins develop, and in the eighth month the vital force (the ''ojas'') is drawn from the mother and to the developing child. The father donates the hard parts of the body to the developing fetus whereas the soft parts come from the mother. Just as with Aristotle, the Sushruta Samhita compares the developing embryo to the clotting of milk into cheese. It claims that conditions of heat result in seven layers of skin being formed around the fetus, just as the creamy layers in cheese form from milk. 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". The Indian medical tradition in the Ayurveda also has conceptions of embryology from antiquity. Then Dalhana, a medieval commentator on the Sushruta Samhita, also describes embryological development. Dalhana claims that in the first month, the fetus has a jelly-like form, whereas cold and heat cause a change to hardness during the second month. Limb differentiation occurs in the third to four months and intelligence even later.


Ancient Greece


Pre-Socratic philosophers

Many 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 Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ph ...
. According to Empedocles (whose views are described by
Plutarch Plutarch (; grc-gre, Πλούταρχος, ''Ploútarchos''; ; – after AD 119) was a Greek Middle Platonist philosopher, historian, biographer, essayist, and priest at the Temple of Apollo in Delphi. He is known primarily for hi ...
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 Alcmaeon of Croton (; el, Ἀλκμαίων ὁ Κροτωνιάτης, ''Alkmaiōn'', ''gen''.: Ἀλκμαίωνος; fl. 5th century BC) was an early Greek medical writer and philosopher-scientist. He has been described as one of the most ...
, says that the milk produced by mammals is analogous to the white of fowl egg. Diogenes of Apollonia claimed that a mass of flesh forms first, only then followed by the development of bone and nerves. Diogenes was able to recognize that the placenta was a nutritional source for the growing fetus. He further claimed 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 being reported to have practiced dissection. One idea which lasted for a long time, first claimed 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. 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''. Greek discussions on embryology often tried to answer several questions. One question regarded whether only the male had a seed which developed into the embryo within the female womb, or whether both the male and the female 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 to answer was the origin of the seed. One theory to answer this, known as 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 regarded 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.James Wilberding, "Plato's Embryology," Early Science and Medicine 2015.


Hippocrates

Some of the most well-known early ideas on embryology come from
Hippocrates Hippocrates of Kos (; grc-gre, Ἱπποκράτης ὁ Κῷος, Hippokrátēs ho Kôios; ), also known as Hippocrates II, was a Greek physician of the classical period who is considered one of the most outstanding figures in the history o ...
and the
Hippocratic Corpus The Hippocratic Corpus (Latin: ''Corpus Hippocraticum''), or Hippocratic Collection, is a collection of around 60 early Ancient Greek medical works strongly associated with the physician Hippocrates and his teachings. The Hippocratic Corpus cov ...
, 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 In the history of biology, preformationism (or preformism) is a formerly popular theory that organisms develop from miniature versions of themselves. Instead of assembly from parts, preformationists believed that the form of living things exist, ...
, 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.


Aristotle

Some embryological discussion appears in the writings of Aristotle's predecessor
Plato Plato ( ; grc-gre, Πλάτων ; 428/427 or 424/423 – 348/347 BC) was a Greek philosopher born in Athens during the Classical period in Ancient Greece. He founded the Platonist school of thought and the Academy, the first institution ...
, especially in his ''
Timaeus Timaeus (or Timaios) is a Greek name. It may refer to: * ''Timaeus'' (dialogue), a Socratic dialogue by Plato *Timaeus of Locri, 5th-century BC Pythagorean philosopher, appearing in Plato's dialogue *Timaeus (historian) (c. 345 BC-c. 250 BC), Greek ...
''. 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. However, a much more voluminous discussion on the subject comes from the writings of
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ph ...
, especially as appears in his '' On the Generation of Animals''. Some ideas related to embryology also appear in his ''
History of Animals ''History of Animals'' ( grc-gre, Τῶν περὶ τὰ ζῷα ἱστοριῶν, ''Ton peri ta zoia historion'', "Inquiries on Animals"; la, Historia Animalium, "History of Animals") is one of the major texts on biology by the ancient Gr ...
'', '' 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.


Later Greek embryology

Reportedly, some Stoics claimed that most parts of the body formed at once during embryological development. Some 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 Herophilos (; grc-gre, Ἡρόφιλος; 335–280 BC), sometimes Latinised Herophilus, was a Greek physician regarded as one of the earliest anatomists. Born in Chalcedon, he spent the majority of his life in Alexandria. He was the first ...
in the 4th 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 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.Samuel Kottek, "Embryology in Talmudic and Midrashic Literature", Journal of the History of Biology 1981. In addition, the Wisdom of Solomon (7:2) also has the embryo formed from menstrual blood.
Soranus of Ephesus Soranus of Ephesus ( grc-gre, Σωρανός ὁ Ἑφέσιος; 1st/2nd century AD) was a Greek physician. He was born in Ephesus but practiced in Alexandria and subsequently in Rome, and was one of the chief representatives of the Methodic ...
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 Aelius Galenus or Claudius Galenus ( el, Κλαύδιος Γαληνός; September 129 – c. AD 216), often Anglicized as Galen () or Galen of Pergamon, was a Greek physician, surgeon and philosopher in the Roman Empire. Considered to be one ...
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 Plutarch (; grc-gre, Πλούταρχος, ''Ploútarchos''; ; – after AD 119) was a Greek Middle Platonist philosopher, historian, biographer, essayist, and priest at the Temple of Apollo in Delphi. He is known primarily for hi ...
has a chapter in one of his works titled "Whether was before, the hen or egg?" Discussion on embryological tradition also appears in many Neoplatonic traditions. 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 In the history of Europe, the Middle Ages or medieval period lasted approximately from the late 5th to the late 15th centuries, similar to the post-classical period of global history. It began with the fall of the Western Roman Empire ...
. 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''. 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.


Patristics

The question of embryology is discussed among a number of patristic authors, 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.Yousef Kouriyhe. "Jakob von Sarug (451-521): Brief an den Erzdiakon Mar Julian — Edessa — 451-521 (Syrisch) — Mekka II — TUK_0955". Corpus Coranicum
Available
Passing reference to the embryo also appears in the eighth hymn of Ephrem the Syrian's ''Paradise Hymns''.) Many patristic treatments of embryology continued in the stream of Greek tradition. 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. 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. Various patristic literature from backgrounds ranging from
Nestorian Nestorianism is a term used in Christian theology and Church history to refer to several mutually related but doctrinarily distinct sets of teachings. The first meaning of the term is related to the original teachings of Christian theologian ...
, Monophysite 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, the Monophysite 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 John Philoponus (Greek: ; ; c. 490 – c. 570), also known as John the Grammarian or John of Alexandria, was a Byzantine Greek philologist, Aristotelian commentator, Christian theologian and an author of a considerable number of philosophical tr ...
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.Dirk Krausmuller, "When Christology intersects with embryology: the viewpoints of Nestorian, Monophysite and Chalcedonian authors of the sixth to tenth centuries", Byzantinische Zeitschrift 2020. Some Monophysites 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 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 Timothy I may refer to: * Pope Timothy I of Alexandria, Pope of Alexandria & Patriarch of the See of St. Mark in 378–384 * Timothy I of Constantinople Timothy I or Timotheus I (? – 1 April 518) was a Christian priest who was appointed Patria ...
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 Jacob of Edessa (or James of Edessa) ( syr, ܝܥܩܘܒ ܐܘܪܗܝܐ, Yaʿqub Urhoyo) (c. 640 – 5 June 708) was Bishop of Edessa and prominent Syriac Christian writer in Classical Syriac language, also known as one of earliest Syriac grammaria ...
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 Moses bar Kepha or Moses bar Cephas (Syriac language, Syriac ''Mushe bar Kipho''; born in Balad, Iraq, Balad in Nineveh, now in Iraq, about the year 813; died at the age of ninety, in 903) was a writer and one of the most celebrated bishops of the ...
claimed, for Christological reasons as a Monophysite, 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.


Embryology in Jewish tradition

Many Jewish authors also discussed notions of embryology, especially as they appear in the
Talmud The Talmud (; he, , Talmūḏ) is the central text of Rabbinic Judaism and the primary source of Jewish religious law ('' halakha'') and Jewish theology. Until the advent of modernity, in nearly all Jewish communities, the Talmud was the ce ...
. 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. his can becompared to milk being put in a vessel: if you add to it some lab-ferment rug or herb it coagulates and stands still; if not, the milk remains liquid." 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 Flavius Josephus (; grc-gre, Ἰώσηπος, ; 37 – 100) was a first-century Romano-Jewish historian and military leader, best known for '' The Jewish War'', who was born in Jerusalem—then part of Roman Judea—to a father of priestly ...
and
Philo of Alexandria Philo of Alexandria (; grc, Φίλων, Phílōn; he, יְדִידְיָה, Yəḏīḏyāh (Jedediah); ), also called Philo Judaeus, was a Hellenistic Jewish philosopher who lived in Alexandria, in the Roman province of Egypt. Philo's de ...
in the 1st century. 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 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.


Embryology in the Islamic tradition

Passing reference to embryological notions also appear in the
Qur'an The Quran (, ; Standard Arabic: , Quranic Arabic: , , 'the recitation'), also romanized Qur'an or Koran, is the central religious text of Islam, believed by Muslims to be a revelation from God. It is organized in 114 chapters (pl.: , s ...
(22:5), where the development of the embryo proceeds in four stages from drop, to a blood clot, to a partially developed stage, to a fully developed child. The notion of clay turning into flesh is seen by some as analogous to a text by Theodoret that describes the same process. The four stages of development in the Qur'an are similar to the four stages of embryological development as described by
Galen Aelius Galenus or Claudius Galenus ( el, Κλαύδιος Γαληνός; September 129 – c. AD 216), often Anglicized as Galen () or Galen of Pergamon, was a Greek physician, surgeon and philosopher in the Roman Empire. Considered to be one ...
. 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. Very similar embryonic descriptions also appear in the Syriac Jacob of Serugh's letter to the Archdeacon Mar Julian. Embryological discussions also appear in the Islamic legal tradition.Ghaly, Mohammed. "Human Embryology in the Islamic Tradition The Jurists of the Post-formative Era in Focus," Islamic Law and Society (2014).


See also

*
Abortion Abortion is the termination of a pregnancy by removal or expulsion of an embryo or fetus. An abortion that occurs without intervention is known as a miscarriage or "spontaneous abortion"; these occur in approximately 30% to 40% of pre ...
* Embryogenesis * Embryonic differentiation waves * Embryology of digestive system and the body cavities * French flag model * Morphogen *
Ontogeny Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the egg to adult. The term can also be used to refer to the s ...
*
Recapitulation theory The theory of recapitulation, also called the biogenetic law or embryological parallelism—often expressed using Ernst Haeckel's phrase "ontogeny recapitulates phylogeny"—is a historical hypothesis that the development of the embryo of an a ...


References


Citations


Sources

* "Germ layer." Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 6 Nov. 2009 . * Welton, John. “, Circle Surrogacy". Circle Surrogacy, Full Service, Worldwide Surrogate-Parenting Agency, www.circlesurrogacy.com/about/what-is-surrogacy.


Further reading

* * Scott F. Gilbert. ''Developmental Biology''. Sinauer, 2003. . * Lewis Wolpert. ''Principles of Development''. Oxford University Press, 2006. . *
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