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Embryology (from Greek ἔμβρυον, ''embryon'', "the unborn,
embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
"; and -λογία, '' -logia'') is the branch of
animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...
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 ...
that studies the prenatal development of gametes (sex cells),
fertilization Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Pro ...
, and development of
embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
s 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, and known as preformationism, 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 ...
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, though accurate observations had been made in Italy by anatomists such as
Aldrovandi Aldrovandi is a family name of the Emilia-Romagna in Italy, and especially famous for the aristocratic and Senatorial family from Bologna. The Palazzo Aldrovandi in central Bologna was built by the family. Among its famous members are: *Cardinal ...
and
Leonardo da Vinci Leonardo di ser Piero da Vinci (15 April 14522 May 1519) was an Italian polymath of the High Renaissance who was active as a painter, draughtsman, engineer, scientist, theorist, sculptor, and architect. While his fame initially rested on ...
in the Renaissance.


Comparative embryology


Preformationism and epigenesis

As recently as the 18th century, the prevailing notion in western human embryology was
preformation 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 exis ...
: 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 ...
. Much early embryology came from the work of the Italian anatomists
Aldrovandi Aldrovandi is a family name of the Emilia-Romagna in Italy, and especially famous for the aristocratic and Senatorial family from Bologna. The Palazzo Aldrovandi in central Bologna was built by the family. Among its famous members are: *Cardinal ...
, Aranzio,
Leonardo da Vinci Leonardo di ser Piero da Vinci (15 April 14522 May 1519) was an Italian polymath of the High Renaissance who was active as a painter, draughtsman, engineer, scientist, theorist, sculptor, and architect. While his fame initially rested on ...
, Marcello Malpighi, Gabriele Falloppio, Girolamo Cardano, Emilio Parisano, Fortunio Liceti, Stefano Lorenzini, Spallanzani, Enrico Sertoli, and Mauro Ruscóni. According to epigenesis, the form of an animal emerges gradually from a relatively formless egg. As microscopy improved during the 19th century, biologists could see that embryos took shape in a series of progressive steps, and epigenesis displaced preformation as the favored explanation among embryologists. 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 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 a cluster of cells the same size ...
), 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 Centrolecithal (''Greek'' kentron = center of a circle, lekithos = yolk) describes the placement of the yolk in the centre of the cytoplasm of ova. Many arthropod eggs are centrolecithal. During cytokinesis, centrolecithal zygotes undergo meroblas ...
).


Basal phyla

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


Bilaterians

In
bilateral animals The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly and ...
cleavage can be either holoblastic or meroblastic depending on the species. During gastrulation the
blastula Blastulation is the stage in early animal embryonic development that produces the blastula. In mammalian development the blastula develops into the blastocyst with a differentiated inner cell mass and an outer trophectoderm. The blastula (fro ...
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 Blastulation is the stage in early animal embryonic development that produces the blastula. In mammalian development the blastula develops into the blastocyst with a differentiated inner cell mass and an outer trophectoderm. The blastula (fro ...
the first pore, or blastopore, becomes the mouth of the animal, it is a
protostome Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryonic development. This nature has since been discovered to be extremely variable among Protostomia's me ...
; if the blastopore becomes the anus then it is a
deuterostome Deuterostomia (; in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some ...
. The
protostomes Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryonic development. This nature has since been discovered to be extremely variable among Protostomia's me ...
include most
invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chorda ...
animals, such as insects, worms and molluscs, while the
deuterostome Deuterostomia (; in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some ...
s include the vertebrates. In due course, the
blastula Blastulation is the stage in early animal embryonic development that produces the blastula. In mammalian development the blastula develops into the blastocyst with a differentiated inner cell mass and an outer trophectoderm. The blastula (fro ...
changes into a more differentiated structure called the gastrula. Soon after the gastrula is formed, three distinct layers of cells (the
germ layers A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans (animals that are sister taxa to the sponges) produce two or three prim ...
) 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). #
Gap gene A gap gene is a type of gene involved in the development of the segmented embryos of some arthropods. Gap genes are defined by the effect of a mutation in that gene, which causes the loss of contiguous body segments, resembling a gap in the nor ...
s establish 3 broad segments of the embryo. #
Pair-rule gene A pair-rule gene is a type of gene involved in the development of the segmented embryos of insects. Pair-rule genes are expressed as a result of differing concentrations of gap gene proteins, which encode transcription factors controlling pair-r ...
s 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 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 The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly and ...
that have holoblastic rotational cleavage. Humans are also
deuterostome Deuterostomia (; in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some ...
s. 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'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 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 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 along with Heinz Christian Pander, also proposed the germ layer theory of development which helped to explain how the embryo developed in progressive steps. Part of this explanation explored why embryos in many species often appear similar to one another in early developmental stages using his four principles.


Modern embryology research

Embryology is central to evolutionary developmental biology ("evo-devo"), which studies the genetic control of the development process (e.g.
morphogen A morphogen is a substance whose non-uniform distribution governs the pattern of tissue development in the process of morphogenesis or pattern formation, one of the core processes of developmental biology, establishing positions of the vario ...
s), its link to cell signalling, its roles in certain diseases and
mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, m ...
s, and its links to
stem cell In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type of ...
research. Embryology is the key to Gestational Surrogacy, which is when the sperm of the intended father and egg of intended mother are fused in a lab forming an embryo. This embryo is then put into the surrogate who carries the child to term.


Medical embryology

Medical embryology is used widely to detect abnormalities before birth. 2-5% of babies are born with an observable abnormality and medical embryology explores the different ways and stages that these abnormalities appear in. Genetically derived abnormalities are referred to as
malformation A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities ca ...
s. When there are multiple malformations, this is considered a syndrome. When abnormalities appear due to outside contributors, these are disruptions. The outside contributors causing disruptions are known as 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 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 phys ...
, 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
embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
s by
Leonardo da Vinci Leonardo di ser Piero da Vinci (15 April 14522 May 1519) was an Italian polymath of the High Renaissance who was active as a painter, draughtsman, engineer, scientist, theorist, sculptor, and architect. While his fame initially rested on ...
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 A medical school is a tertiary educational institution, or part of such an institution, that teaches medicine, and awards a professional degree for physicians. Such medical degrees include the Bachelor of Medicine, Bachelor of Surgery (MBBS, ...
, 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 Akhenaten (pronounced ), also spelled Echnaton, Akhenaton, ( egy, ꜣḫ-n-jtn ''ʾŪḫə-nə-yātəy'', , meaning "Effective for the Aten"), was an ancient Egyptian pharaoh reigning or 1351–1334 BC, the tenth ruler of the Eighteenth D ...
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 The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later fetus develops in amniotes. It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until sh ...
appear in the '' Bhagavad Gita,
Bhagavata Purana The ''Bhagavata Purana'' ( sa, भागवतपुराण; ), also known as the ''Srimad Bhagavatam'', ''Srimad Bhagavata Mahapurana'' or simply ''Bhagavata'', is one of Hinduism's eighteen great Puranas (''Mahapuranas''). Composed in S ...
'', 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 Chyle (from the Greek word χυλός ''chylos'', "juice") is a milky bodily fluid consisting of lymph and emulsified fats, or free fatty acids (FFAs). It is formed in the small intestine during digestion of fatty foods, and taken up by lymph ...
. 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 The Upanishads (; sa, उपनिषद् ) are late Vedic Sanskrit texts that supplied the basis of later Hindu philosophy.Wendy Doniger (1990), ''Textual Sources for the Study of Hinduism'', 1st Edition, University of Chicago Press, , ...
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 Ayurveda () is an alternative medicine system with historical roots in the Indian subcontinent. The theory and practice of Ayurveda is pseudoscientific. Ayurveda is heavily practiced in India and Nepal, where around 80% of the population rep ...
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 Pre-Socratic philosophy, also known as early Greek philosophy, is ancient Greek philosophy before Socrates. Pre-Socratic philosophers were mostly interested in cosmology, the beginning and the substance of the universe, but the inquiries of th ...
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 ...
. According to
Empedocles Empedocles (; grc-gre, Ἐμπεδοκλῆς; , 444–443 BC) was a Greek pre-Socratic philosopher and a native citizen of Akragas, a Greek city in Sicily. Empedocles' philosophy is best known for originating the cosmogonic theory of the ...
(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, says that the milk produced by mammals is analogous to the white of fowl egg.
Diogenes of Apollonia Diogenes of Apollonia ( ; grc, Διογένης ὁ Ἀπολλωνιάτης, Diogénēs ho Apollōniátēs; 5th century BC) was an ancient Greek philosopher, and was a native of the Milesian colony Apollonia in Thrace. He lived for some t ...
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 Democritus (; el, Δημόκριτος, ''Dēmókritos'', meaning "chosen of the people"; – ) was an Ancient Greek pre-Socratic philosopher from Abdera, primarily remembered today for his formulation of an atomic theory of the universe. No ...
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, where discussion on the embryo is usually given in the context of discussing
obstetrics Obstetrics is the field of study concentrated on pregnancy, childbirth and the postpartum period. As a medical specialty, obstetrics is combined with gynecology under the discipline known as obstetrics and gynecology (OB/GYN), which is a sur ...
(pregnancy and childbirth). Some of the most relevant Hippocratic texts on embryology include the ''Regimen on Acute Diseases'', ''On Semen'', and ''On the Development of the Child''. Hippocrates claimed that the development of the embryo is put into motion by fire and that nourishment comes from food and breath introduced into the mother. An outer layer of the embryo solidifies, and the fire within consumes humidity which makes way for development of bone and nerve. The fire in the innermost part becomes the belly and air channels are developed in order to route nourishment to it. The enclosed fire also helps form veins and allows for circulation. In this description, Hippocrates aims at describing the causes of development rather than describing what develops. Hippocrates also develops views similar to preformationism, where he claims that all parts of the embryo simultaneously develop. Hippocrates also believed that maternal blood nourishes the embryo. This blood flows and coagulates to help form the flesh of the fetus. This idea was derived from the observation that menstrual blood ceases during pregnancy, which Hippocrates took to imply that it was being redirected to fetal development. Hippocrates also claimed that the flesh differentiates into different organs of the body, and Hippocrates saw as analogous an experiment where a mixture of substances placed into water will differentiate into different layers. Comparing the seed to the embryo, Hippocrates further compared the stalk to the umbilical cord.


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''. 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 ...
, especially as appears in his ''
On the Generation of Animals The ''Generation of Animals'' (or ''On the Generation of Animals''; Greek: ''Περὶ ζῴων γενέσεως'' (''Peri Zoion Geneseos''); Latin: ''De Generatione Animalium'') is one of the biological works of the Corpus Aristotelicum, the co ...
''. 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 Aristotle's biology, texts on biolo ...
'', '' 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 Aeschylus (, ; grc-gre, Αἰσχύλος ; c. 525/524 – c. 456/455 BC) was an ancient Greek tragedian, and is often described as the father of tragedy. Academic knowledge of the genre begins with his work, and understanding of earlier Gree ...
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 Melanesians are the predominant and indigenous inhabitants of Melanesia, in a wide area from Indonesia's New Guinea to as far East as the islands of Vanuatu and Fiji. Most speak either one of the many languages of the Austronesian language fam ...
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 Stoicism is a school of Hellenistic philosophy founded by Zeno of Citium in Athens in the early 3rd century BCE. It is a philosophy of personal virtue ethics informed by its system of logic and its views on the natural world, asserting tha ...
claimed that most parts of the body formed at once during embryological development. Some
Epicureans Epicureanism is a system of philosophy founded around 307 BC based upon the teachings of the ancient Greek philosopher Epicurus. Epicureanism was originally a challenge to Platonism. Later its main opponent became Stoicism. Few writings by ...
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 Tertullian (; la, Quintus Septimius Florens Tertullianus; 155 AD – 220 AD) was a prolific early Christian author from Carthage in the Roman province of Africa. He was the first Christian author to produce an extensive corpus of L ...
, Herophilus 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 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 o ...
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 Ephrem the Syrian ( syc, ܡܪܝ ܐܦܪܝܡ ܣܘܪܝܝܐ, Mār ʾAp̄rêm Sūryāyā, ; grc-koi, Ἐφραὶμ ὁ Σῦρος, Efrém o Sýros; la, Ephraem Syrus; am, ቅዱስ ኤፍሬም ሶርያዊ; ), also known as Saint Ephrem, Saint ...
'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 Tertullian (; la, Quintus Septimius Florens Tertullianus; 155 AD – 220 AD) was a prolific early Christian author from Carthage in the Roman province of Africa. He was the first Christian author to produce an extensive corpus of L ...
held that the soul was present from the moment of conception. The
Quinisext Council The Quinisext Council (Latin: ''Concilium Quinisextum''; Koine Greek: , ''Penthékti Sýnodos''), i.e. the Fifth-Sixth Council, often called the Council ''in Trullo'', Trullan Council, or the Penthekte Synod, was a church council held in 692 at ...
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 Augustine of Hippo ( , ; la, Aurelius Augustinus Hipponensis; 13 November 354 – 28 August 430), also known as Saint Augustine, was a theologian and philosopher of Berber origin and the bishop of Hippo Regius in Numidia, Roman North Afr ...
, 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, 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 Origen of Alexandria, ''Ōrigénēs''; Origen's Greek name ''Ōrigénēs'' () probably means "child of Horus" (from , "Horus", and , "born"). ( 185 – 253), also known as Origen Adamantius, was an early Christian scholar, ascetic, and the ...
, 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 Gregory of Nyssa, also known as Gregory Nyssen ( grc-gre, Γρηγόριος Νύσσης; c. 335 – c. 395), was Bishop of Nyssa in Cappadocia from 372 to 376 and from 378 until his death in 395. He is venerated as a saint in Catholicis ...
, 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 Philoxenus of Mabbug (Syriac: , ') (died 523), also known as Xenaias and Philoxenus of Hierapolis, was one of the most notable Syriac prose writers and a vehement champion of Miaphysitism. Early life He was born, probably in the third quarter of ...
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 Leontius ( el, Λεόντιος, Leóntios; – 15 February 706), was Byzantine emperor from 695 to 698. Little is known of his early life, other than that he was born in Isauria in Asia Minor. He was given the title of ''patrikios'', and mad ...
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 Adoptionism, also called dynamic monarchianism, is an early Christian nontrinitarian theological doctrine, which holds that Jesus was adopted as the Son of God at his baptism, his resurrection, or his ascension. How common adoptionist views ...
.
Maximus the Confessor Maximus the Confessor ( el, Μάξιμος ὁ Ὁμολογητής), also spelt Maximos, otherwise known as Maximus the Theologian and Maximus of Constantinople ( – 13 August 662), was a Christian monk, theologian, and scholar. In his ea ...
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 argued that the Word first united with the body, and only later with the soul. He cited John 1:1, claiming on its basis that the Word became flesh first, not a human being first. Then, Jacob of Edessa rejected ''prohyparxis'' because Origen had defended it and ''methyparxis'' because he believed that it made the soul ontologically inferior and as only being made for the body. Then, Moses Bar Kepha claimed, for Christological reasons as a 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 The Book of Job (; hbo, אִיּוֹב, ʾIyyōḇ), or simply Job, is a book found in the Ketuvim ("Writings") section of the Hebrew Bible (Tanakh), and is the first of the Poetic Books in the Old Testament of the Christian Bible. Scholars a ...
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 ''Midrash'' (;"midrash"
''Random House Webster's Unabridged Dictionary''.
he, מִדְרָשׁ; ...
: "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 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.: , si ...
(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 o ...
. In the early 6th century,
Sergius of Reshaina Sergius of Reshaina (died 536) was a physician and priest during the 6th century. He is best known for translating medical works from Greek to Syriac, which were eventually, during the Abbasid Caliphate of the late 8th- & 9th century, translated in ...
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 Gundeshapur ( pal, 𐭥𐭧𐭩𐭠𐭭𐭣𐭩𐭥𐭪𐭱𐭧𐭯𐭥𐭧𐭥𐭩, ''Weh-Andiōk-Šābuhr''; New Persian: , ''Gondēshāpūr'') was the intellectual centre of the Sassanid Empire and the home of the Academy of Gundishapur, founded ...
, known as the
Academy of Gondishapur The Academy of Gondishapur ( fa, فرهنگستان گندی‌شاپور, Farhangestân-e Gondišâpur), also known as the Gondishapur University (دانشگاه گندی‌شاپور Dânešgâh-e Gondišapur), was one of the three Sasanian ...
, 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 An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
* Embryonic differentiation waves * Embryology of digestive system and the body cavities * French flag model *
Morphogen A morphogen is a substance whose non-uniform distribution governs the pattern of tissue development in the process of morphogenesis or pattern formation, one of the core processes of developmental biology, establishing positions of the vario ...
*
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


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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