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Embryology is the study of the development of an embryo. An embryo is defined as any organism in a stage before birth or hatching, or in plants, before germination occurs.

Embryology refers to the development of the fertilized egg cell (zygote) and its differentiation into tissues and organs. After cleavage, the dividing cells, or morula, becomes a hollow ball, or blastula, which develops a hole or pore at one end.

In animals, the blastula develops in one of two ways that divides the whole animal kingdom into two halves. If in the blastula the first pore (blastopore) becomes the mouth of the animal, it is a protostome; if the first pore becomes the anus then it is a deuterostome. The protostomes include most invertebrate animals, such as insects, worms and molluscs, while the deuterostomes includes more advanced animals including the vertebrates. In due course, the blastula changes into a more differentiated structure called the gastrula.

The gastrula with its blastopore soon develops three distinct layers of cells (the germ layers) from which all the bodily organs and tissues then develop:

• The innermost layer, or endoderm, gives rise to the digestive organs, lungs and bladder.
• The middle layer, or mesoderm, gives rise to the muscles, skeleton and blood system.
• The outer layer of cells, or ectoderm, gives rise to the nervous system and skin.

In humans, the term embryo refers to the ball of dividing cells from the moment the zygote implants itself in the uterus wall until the end of the eighth week after conception. Beyond the eighth week, the developing human is then called a fetus.

Contents 1 History 2 Vertebrate and invertebrate embryology 3 Modern embryology research 4 See also 5 Footnotes 6 References 7 Further reading 8 External links

With a very long history, embryology is a subject that has engaged the minds of some of the most brilliant and original biological thinkers and experimentalists.

In modern times, these include Gavin de Beer, Charles Darwin, Ernst Haeckel, J.B.S. Haldane, and Joseph Needham, while much early embryology came from the work of Aristotle and the great Italian anatomists: Aldrovandi, Aranzio, Leonardo da Vinci, Marcello Malpighi, Gabriele Falloppia, Girolamo Cardano, Emilio Parisano, Fortunio Liceti, Stefano Lorenzini, Spallanzani, Enrico Sertoli, Mauro Rusconi, etc.^[1] Other important contributors include William Harvey, Kaspar Friedrich Wolff, Pander, Karl Ernst von Baer, and August Weismann.

After the 1950s, with the DNA helical structure being unravelled 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.

Many principles of embryology apply to both invertebrate animals as well as to vertebrates.^[2] Therefore, the study of invertebrate embryology has advanced the study of vertebrate embryology. However, there are many differences as well. For example, numerous invertebrate species release a larva before development is complete; at the end of the larval period, an animal for the first time comes to resemble an adult similar to it 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.

Currently, embryology has become an important research area for studying the genetic control of the development process (e.g. morphogens), its link to cell signalling, its importance for the study of certain diseases and mutations and in links to stem cell research.

• Ontogeny
• Embryogenesis
• Recapitulation theory
• Prenatal development
• Protostomes
• Deuterostomes
• Germ layers
• Epigenesis (biology)
• Developmental biology
• Cell signalling
• Hedgehog signaling pathway
• Morphogens

1. ^ http://www.ijdb.ehu.es/fullaccess/fulltext.0009/ft515.pdf Massimo De Felici, Gregorio Siracus, The rise of embryology in Italy: from the Renaissance to the early 20th Century, Int. J. Dev. Biol. 44: 515-521 (2000)
2. ^ Parker, Sybil. "Invertebrate Embryology," McGraw-Hill Encyclopedia of Science & Technology (McGraw-Hill 1997).

• UNSW Embryology Large resource of information and media
• [1] Definition of embryo according to Webster

• Scott F. Gilbert. Developmental Biology. Sinauer, 2003. ISBN 0-87893-258-5.
• Lewis Wolpert. Principles of Development. Oxford University Press, 2006. ISBN 0-19-927536-X.

• Embryo Research UK philosophy and ethics website discussing the ethics of embryology
• Human embryo research Canadian website covering the ethics of human embryo research
• University of Indiana's Human Embryology Animations

v • d • e Developmental Biology > Human embryogenesis (development of embryo) and development of fetus (some dates are approximate - see Carnegie stages and a timeline)
Week 1	Fertilization - Egg activation - Zygote - Cleavage - Morula - Blastula (Blastomere) - Blastocyst - Inner cell mass
Week 2	Bilaminar disc (Hypoblast, Epiblast)
Week 3 (Trilaminar embryo, germ layers)	Archenteron/Primitive streak (Primitive pit, Primitive knot/Blastopore, Primitive groove) - Gastrula/Gastrulation - Regional specification Ectoderm: Surface ectoderm - Neuroectoderm - Somatopleure - Neurulation - Neural crest Endoderm: Splanchnopleure Mesoderm: Chorda- - Paraxial (Somite/Somitomere/Sclerotome/Myotome/Dermatome) - Intermediate - Lateral plate (Intraembryonic coelom, Splanchnopleure/Somatopleure)
Extraembryonic/uterus	Trophoblast (Cytotrophoblast, Syncytiotrophoblast) Blastocoele - Yolk sack/exocoelomic cavity - Heuser's membrane - Extraembryonic coelom - Vitelline duct Umbilical cord (Umbilical artery, Umbilical vein, Wharton's jelly) - Allantois Placenta - Decidua (Decidual cells) - Chorionic villi/Intervillous space - Gestational sac (Amnion/Amniotic sac/Amniotic cavity, Chorion)
Histogenesis	Programmed cell death - Stem cells - Germ line development
Organogenesis	Limb development: Limb bud - Apical ectodermal ridge/AER - Eye development - Cutaneous structure development - Heart development - Development of the urinary and reproductive organs
Post Embryonic Development	Metamorphosis - Regeneration - Aging

v • d • e Prenatal development/Mammalian development of digestive system
Gut	Stomodeum Foregut (Buccopharyngeal membrane, Rathke's pouch, Tracheoesophageal septum, Pancreatic bud, Hepatic diverticulum) Midgut Hindgut (Urorectal septum ) Proctodeum
Mesentery	Dorsal mesentery - Ventral mesentery
Other	Septum transversum

v • d • e Prenatal development/mammalian embryogenesis - Development of the urinary and reproductive organs
General Urinary/Reproductive system	Cloacal membrane - Cloaca - Urethral groove - Urogenital sinus - Urachus - Urogenital folds
Kidney development	Nephrogenic cord - Nephrotome - Pronephros - Mesonephros (Mesonephric tubules) Ureteric bud - Metanephric blastema
Fetal genital development - primarily internal	Gonadal ridge - Sex cord (Cortical cords, Testis cords) Pronephric duct/Wolffian duct/mesonephric duct - Müllerian duct/paramesonephric ducts (Vaginal plate)
Primarily external	Genital tubercle - Phallus - Labioscrotal folds Gubernaculum - Processus vaginalis
see also list of homologues of the human reproductive system

v • d • e Prenatal development/Mammalian development of nervous system
General neural development/ neurulation/neurula	Notochord - Neuroectoderm - Neural plate - Neural folds - Neural groove Neural crest - Neural tube (Neuromere/Rhombomere, Cephalic flexure) Alar plate - Basal plate
Eye development	Optic vesicles - Optic stalk - Optic cup - Lens placode
Auditory development	Auditory vesicle - Auditory pit

v • d • e Prenatal development/Mammalian development of circulatory system
Vascular	Blood island arteries: Dorsal aorta - Aortic arches - Vitelline arteries - Ductus arteriosus - Umbilical artery veins: Cardinal veins - Ducts of Cuvier - Vitelline veins - Ductus venosus - Umbilical vein
Heart development	Primitive heart tube: Truncus arteriosus - Bulbus cordis - Primitive ventricle - Primitive atrium - Sinus venosus Septum primum (Ostium primum, Ostium secundum) - Septum secundum (Foramen ovale) - other septa (Endocardial cushions/Septum intermedium, Aorticopulmonary septum) - Atrial canal

v • d • e Prenatal development/Mammalian development of respiratory system (overview)
Upper	Nasal placode
Lower	Laryngotracheal groove - Lung buds

v • d • e Human development of head and neck
Branchial region	Branchial arch (1st/Meckel's cartilage, 2nd)/Branchial pouch - Cervical sinus - Lateral lingual swelling - Tuberculum impar - Frontonasal prominence - Olfactory pit - Nasal processes - Globular process - Primitive palate - Maxillary prominence
Tooth development	Dental papilla - Odontoblast - Ameloblast
Other	Thyroglossal duct