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104-540: Well-known biologist, explorer and founder of embryology , Karl Ernst von Baer (1792–1876), was born in Piibe Manor and owned the complex from 1834 to 1866. 58°57′N 26°09′E  /  58.950°N 26.150°E  / 58.950; 26.150 This Lääne-Viru County location article is a stub . You can help Misplaced Pages by expanding it . Embryology Embryology (from Greek ἔμβρυον, embryon , "the unborn, embryo "; and -λογία, -logia )

208-547: 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. 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

312-464: 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 . Galen discusses his understanding of embryology in two of his texts, those being his On

416-483: 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. Judaism allows assisted reproduction, such as IVF embryo transfer and maternal surrogacy, when

520-443: A cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a compact mass called the morula . Cleavage ends with the formation of the blastula , or of the blastocyst in mammals. Depending mostly on the concentration of yolk in the egg, the cleavage can be holoblastic (total or complete cleavage) or meroblastic (partial or incomplete cleavage). The pole of

624-422: 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. Humans are bilateral animals that have holoblastic rotational cleavage. Humans are also deuterostomes . In regard to humans, the term embryo refers to

728-415: A gradient)— bilateral holoblastic, radial holoblastic, rotational holoblastic, and spiral holoblastic, cleavage. These holoblastic cleavage planes pass all the way through isolecithal zygotes during the process of cytokinesis. Coeloblastula is the next stage of development for eggs that undergo these radial cleavages. In holoblastic eggs, the first cleavage always occurs along the vegetal-animal axis of

832-574: 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

936-406: 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. After the 1950s, with

1040-686: A polar lobe. This polar lobe forms at the vegetal pole during cleavage, and then gets shunted to the D blastomere. The polar lobe contains vegetal cytoplasm, which becomes inherited by the future D macromere. In the presence of a large concentration of yolk in the fertilized egg cell, the cell can undergo partial, or meroblastic, cleavage. Two major types of meroblastic cleavage are discoidal and superficial . A. Isolecithal (sparse, evenly distributed yolk) B. Mesolecithal (moderate vegetal yolk disposition) A. Telolecithal (dense yolk throughout most of cell) B. Centrolecithal (yolk in center of egg) Compared to other fast developing animals, mammals have

1144-400: 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

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1248-557: 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

1352-434: A slower rate of division that is between 12 and 24 hours. Initially synchronous, these cellular divisions progressively become more and more asynchronous. Zygotic transcription starts at the two-, four-, or eight-cell stage depending on the species (for example, mouse zygotic transcription begins towards the end of the zygote stage and becomes significant at the two-cell stage, whereas human embryos begin zygotic transcription at

1456-406: Is alternating symmetry between the odd and even quartets. In other words, the orientation of divisions that produces each quartet alternates between being clockwise and counterclockwise with respect to the animal pole. The alternating cleavage pattern that occurs as the quartets are generated produces quartets of micromeres that reside in the cleavage furrows of the four macromeres. When viewed from

1560-410: Is considered a syndrome . 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. Many principles of embryology apply to invertebrates as well as to vertebrates. Therefore, the study of invertebrate embryology has advanced

1664-542: 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 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

1768-497: Is made up of a central spindle and polar asters made up of polymers of tubulin protein called microtubules . The asters are nucleated by centrosomes and the centrosomes are organized by centrioles brought into the egg by the sperm as basal bodies . Cytokinesis is mediated by the contractile ring made up of polymers of actin protein called microfilaments . Karyokinesis and cytokinesis are independent but spatially and temporally coordinated processes. While mitosis can occur in

1872-486: 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. The question of embryology is discussed among a number of early Christian writers , largely in terms of theological questions such as whether

1976-469: Is taught as a cornerstone subject in medical schools , as well as in biology and zoology programs at both an undergraduate and graduate level. 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

2080-478: Is the branch of animal biology that studies the prenatal development of gametes (sex cells), fertilization , and development of embryos and fetuses . 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

2184-566: 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 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

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2288-453: 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. As of today, human embryology

2392-658: 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

2496-622: 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

2600-415: The cells being set early in the embryo development . Each blastomere produced by early embryonic cleavage does not have the capacity to develop into a complete embryo . A cell can only be indeterminate (also called regulative) if it has a complete set of undisturbed animal/vegetal cytoarchitectural features. It is characteristic of deuterostomes —when the original cell in a deuterostome embryo divides,

2704-401: The deuterostomes 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. Drosophila have been used as

2808-401: 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 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,

2912-552: 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. 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

3016-526: The 13th postnatal day. 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

3120-492: The 16–cell stage the compacted embryo is called a morula . Once the embryo has divided into 16 cells, it begins to resemble a mulberry , hence the name morula ( Latin , morus : mulberry ). Concomitantly, they develop an inside-out polarity that provides distinct characteristics and functions to their cell-cell and cell-medium interfaces. As surface cells become epithelial , they begin to tightly adhere as gap junctions are formed, and tight junctions are developed with

3224-685: 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. Meroblastic In embryology , cleavage is the division of cells in the early development of the embryo , following fertilization . The zygotes of many species undergo rapid cell cycles with no significant overall growth, producing

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3328-466: 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 , 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

3432-718: 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

3536-499: 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 Monophysite, that only synhyparxis was acceptable. He claimed that Genesis 2:7 has no temporal sequence and that Exodus 21:22 regards

3640-417: The absence of cytokinesis, cytokinesis requires the mitotic apparatus. The end of cleavage coincides with the beginning of zygotic transcription. This point in non-mammals is referred to as the midblastula transition and appears to be controlled by the nuclear-cytoplasmic ratio (about 1:6). Determinate cleavage (also called mosaic cleavage) is in most protostomes . It results in the developmental fate of

3744-403: The animal pole, this arrangement of cells displays a spiral pattern. Specification of the D macromere and is an important aspect of spiralian development. Although the primary axis, animal-vegetal, is determined during oogenesis , the secondary axis, dorsal-ventral, is determined by the specification of the D quadrant. The D macromere facilitates cell divisions that differ from those produced by

3848-524: 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 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

3952-423: 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 ). In bilateral animals , cleavage can be either holoblastic or meroblastic depending on

4056-507: 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

4160-527: 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

4264-460: 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

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4368-461: The cleavage spindle. This occurs when the aster at one pole attaches to the cell membrane, causing it to be much smaller than the aster at the other pole. This results in an unequal cytokinesis , in which both macromeres inherit part of the animal region of the egg, but only the bigger macromere inherits the vegetal region. The second mechanism of unequal cleavage involves the production of an enucleate, membrane bound, cytoplasmic protrusion, called

4472-420: 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 ,

4576-422: The divisions of the blastomeres are complete and separate; compared with bilateral meroblastic cleavage, in which the blastomeres stay partially connected. Radial cleavage is characteristic of the deuterostomes , which include some vertebrates and echinoderms , in which the spindle axes are parallel or at right angles to the polar axis of the oocyte . Rotational cleavage involves a normal first division along

4680-620: The egg that grows in women. A variety of conceptions on embryology appeared in ancient Asia. Descriptions of the amniotic sac appear in the Bhagavad Gita , Bhagavata Purana , 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

4784-441: The egg with the highest concentration of yolk is referred to as the vegetal pole while the opposite is referred to as the animal pole . Cleavage differs from other forms of cell division in that it increases the number of cells and nuclear mass without increasing the cytoplasmic mass. This means that with each successive subdivision, there is roughly half the cytoplasm in each daughter cell than before that division, and thus

4888-437: The egg, the second cleavage is perpendicular to the first. From here, the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms. The first cleavage results in bisection of the zygote into left and right halves. The following cleavage planes are centered on this axis and result in the two halves being mirror images of one another. In bilateral holoblastic cleavage,

4992-423: The eight-cell stage). Cleavage is holoblastic and rotational. In human embryonic development at the eight-cell stage, having undergone three cleavages the embryo starts to change shape as it develops into a morula and then a blastocyst . At the eight-cell stage the blastomeres are initially round, and only loosely adhered. With further division in the process of compaction the cells flatten onto one another. At

5096-481: 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

5200-417: The embryo by hydraulic fracturing . Initially dispersed in hundreds of water pockets throughout the embryo, the fluid collects into a single large cavity , called blastocoel, following a process akin to Ostwald ripening . Embryoblast cells also known as the inner cell mass form a compact mass of cells at the embryonic pole on one side of the cavity that will go on to produce the embryo proper. The embryo

5304-407: 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." 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

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5408-480: 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

5512-493: 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

5616-573: 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

5720-538: 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 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 ). Animals that belong to

5824-457: 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

5928-411: 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

6032-399: 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

6136-511: 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

6240-420: The fetus has conscious experiences towards the end of its development. 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

6344-499: 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 . 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

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6448-445: 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,

6552-440: 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

6656-413: The first two cell divisions result in four macromeres, also called blastomeres, (A, B, C, D) each representing one quadrant of the embryo. These first two cleavages are not oriented in planes that occur at right angles parallel to the animal-vegetal axis of the zygote . At the 4-cell stage, the A and C macromeres meet at the animal pole, creating the animal cross-furrow, while the B and D macromeres meet at

6760-404: 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. Some of

6864-414: 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 is the very beginning steps of a developing embryo. Cleavage refers to the many mitotic divisions that occur after

6968-447: 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. Many Jewish authors also discussed notions of embryology, especially as they appear in the Talmud . Much of

7072-449: 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

7176-419: The formation of the third quartet, one of the macromeres initiates maximum contact with the overlying micromeres in the animal pole of the embryo. This contact is required to distinguish one macromere as the official D quadrant blastomere. In equally cleaving spiral embryos, the D quadrant is not specified until after the formation of the third quartet, when contact with the micromeres dictates one cell to become

7280-410: 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

7384-509: The future D blastomere. Once specified, the D blastomere signals to surrounding micromeres to lay out their cell fates. In unequal cleavage , the first two cell divisions are unequal producing four cells in which one cell is bigger than the other three. This larger cell is specified as the D macromere. Unlike equally cleaving spiralians, the D macromere is specified at the four-cell stage during unequal cleavage. Unequal cleavage can occur in two ways. One method involves asymmetric positioning of

7488-537: 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

7592-470: 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

7696-454: 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

7800-641: The meridional axis, giving rise to two daughter cells. The way in which this cleavage differs is that one of the daughter cells divides meridionally, whilst the other divides equatorially. The nematode C. elegans , a popular developmental model organism , undergoes holoblastic rotational cell cleavage. Spiral cleavage is conserved between many members of the lophotrochozoan taxa, referred to as Spiralia . Most spiralians undergo equal spiral cleavage, although some undergo unequal cleavage (see below). This group includes annelids , molluscs , and sipuncula . Spiral cleavage can vary between species, but generally

7904-439: The morula enters the uterus after three or four days, and begins to take in fluid, as sodium-potassium pumps on the trophoblasts pump sodium into the morula, drawing in water by osmosis from the maternal environment to become blastocoelic fluid. As a consequence to increased osmotic pressure, the accumulation of fluid raises the hydrostatic pressure inside the embryo. Hydrostatic pressure breaks open cell-cell contacts within

8008-562: 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

8112-488: 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

8216-622: 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

8320-413: The other blastomeres. With further compaction the individual outer blastomeres, the trophoblasts , become indistinguishable as they become organised into a thin sheet of tightly adhered epithelial cells . They are still enclosed within the zona pellucida . The morula is now watertight, to contain the fluid that the cells will later pump into the embryo to transform it into the blastocyst. In humans,

8424-421: The other three macromeres. Cells of the D quadrant give rise to dorsal and posterior structures of the spiralian. Two known mechanisms exist to specify the D quadrant. These mechanisms include equal cleavage and unequal cleavage. In equal cleavage , the first two cell divisions produce four macromeres that are indistinguishable from one another. Each macromere has the potential of becoming the D macromere. After

8528-428: 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 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

8632-407: 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

8736-479: The ratio of nuclear to cytoplasmic material increases. The rapid cell cycles are facilitated by maintaining high levels of proteins that control cell cycle progression such as the cyclins and their associated cyclin-dependent kinases (CDKs). The complex cyclin B / CDK1 also known as MPF ( maturation promoting factor ) promotes entry into mitosis. The processes of karyokinesis (mitosis) and cytokinesis work together to result in cleavage. The mitotic apparatus

8840-507: The relationship between groups can be determined based upon common embryonic and larval structures. 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 are those that appear to be similar but have no common ancestral derivation. Until

8944-483: The same process. The three veils of darkness are described in commentary of the Qur'an as the anterior abdominal wall , the uterine wall and the amniochorionic membrane . 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

9048-400: The soul pre-existed the body. 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

9152-460: 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

9256-462: The spermatozoon and oocyte originate from the respective husband and wife. 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. The notion of clay turning into flesh is seen by some as analogous to a text by Theodoret that describes

9360-564: 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,

9464-651: The subject comes from the writings of Aristotle , especially as appears in his On the Generation of Animals . 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

9568-450: 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. Reportedly, some Stoics claimed that most parts of the body formed at once during embryological development. Some Epicureans claimed that

9672-522: 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 , though accurate observations had been made in Italy by anatomists such as Aldrovandi and Leonardo da Vinci in the Renaissance. As recently as the 18th century, the prevailing notion in western human embryology

9776-555: The third month. 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 . 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

9880-502: The two resulting cells can be separated, and each one can individually develop into a whole organism. In holoblastic cleavage, the zygote and blastomeres are completely divided during the cleavage, so the number of blastomeres doubles with each cleavage. In the absence of a large concentration of yolk, four major cleavage types can be observed in isolecithal cells (cells with a small, even distribution of yolk) or in mesolecithal cells or microlecithal cells (moderate concentration of yolk in

9984-493: The vegetal pole, creating the vegetal cross-furrow. With each successive cleavage cycle, the macromeres give rise to quartets of smaller micromeres at the animal pole. The divisions that produce these quartets occur at an oblique angle, an angle that is not a multiple of 90 degrees, to the animal-vegetal axis. Each quartet of micromeres is rotated relative to their parent macromere, and the chirality of this rotation differs between odd- and even-numbered quartets, meaning that there

10088-413: The writings of Aristotle's predecessor Plato , especially in his 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. However, a much more voluminous discussion on

10192-650: 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 . Much early embryology came from the work of the Italian anatomists 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,

10296-410: 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. Some embryological discussion appears in

10400-440: 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

10504-465: 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

10608-644: 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. In addition,

10712-422: 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

10816-413: 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

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