Endoderm is one of the germ layers-- aggregates of cells that organize early during embryonic life and from which all organs and tissues develop. All animals, with the exception of sponges, form either two or three germ layers through a process known as gastrulation. During gastrulation, a ball of cells transforms into a two-layered embryo made of an inner layer of endoderm and an outer layer of ectoderm. In more complex organisms, like vertebrates, these two primary germ layers interact to give rise to a third germ layer, called mesoderm. Regardless of the presence of two or three layers, endoderm is always the inner-most layer. Endoderm forms the epithelium-- a type of tissue in which the cells are tightly linked together to form sheets-- that lines the primitive gut. From this epithelial lining of the primitive gut, organs like the digestive tract, liver, pancreas, and lungs develop.

Felix Anton Dohrn is best remembered as the founder of the Stazione Zoologica di Napoli, the world' s first permanent laboratory devoted to the study of marine organisms. Dohrn was born on 29 December 1840 in Stettin, Pomerania (now Poland), to a wealthy merchant family. Dohrn's paternal grandfather, Heinrich, trained as a surgeon and then established a sugar refinery, while Dohrn's father, Carl August Dohrn, who inherited the family business, became interested in natural history through Alexander von Humboldt, a family friend. Once settled in his career, Anton Dohrn's own research never strayed far from the origin of vertebrates. He promoted the theory that vertebrates closely resemble and are derived from annelid-like ancestors and he spent years studying the early embryogenesis of lower vertebrates in attempts to prove this.

A germ layer is a group of cells in an embryo that interact with each other as the embryo develops and contribute to the formation of all organs and tissues. All animals, except perhaps sponges, form two or three germ layers. The germ layers develop early in embryonic life, through the process of gastrulation. During gastrulation, a hollow cluster of cells called a blastula reorganizes into two primary germ layers: an inner layer, called endoderm, and an outer layer, called ectoderm. Diploblastic organisms have only the two primary germ layers; these organisms characteristically have multiple symmetrical body axes (radial symmetry), as is true of jellyfish, sea anemones, and the rest of the phylum Cnidaria. All other animals are triploblastic, as endoderm and ectoderm interact to produce a third germ layer, called mesoderm. Together, the three germ layers will give rise to every organ in the body, from skin and hair to the digestive tract.

Etienne Geoffroy Saint-Hilaire, commonly known as Geoffroy, studied animals, their anatomy and their embryos, and teratogens at the National Museum of Natural History in Paris, France in the eighteenth and nineteenth centuries. Geoffroy also helped develop several specialized fields in the life sciences, including experimental embryology. In his efforts to experimentally demonstrate the theory of recapitulation, Geoffroy developed techniques to intervene in the growth of embryos to see whether they would develop into different kinds of organisms. Moreover, Geoffroy emphasized the concept of l'unite de composition (the unity of plan). Geoffroy disputed in 1830 with Georges Cuvier over whether form or function matters most for the study of anatomy and whether the transformation of organic forms can occur over time. Geoffroy's conceptual contributions, as well as his experimental research, influenced embryological research on animal morphology and teratogens, and later the field of evolutionary paleontology.

This diagram shows how NCCs migrate differently in rats, birds and amphibians. The arrows represent both chronology of NCCs migration and the differential paths that NCCs follow in different classes of animals. The solid black portion of each illustration represents the neural crest, and the large black dots in (c) and in (f) represent the neural crest cells. The speckled sections that at first form a basin in (a) and then close to form a tube in (f) represent the neural ectoderm. The solid white portions represent the epidermal ectoderm. During the neurula stage of all vertebrate embryos (a), the neural crest is located in two places on the neural plate. As the neural tube forms (b), a process called neurulation, the neural crest moves with the folding plate as it forms the junction between the neural and epidermal ectoderm. NCCs migrate differently in different classes of vertebrates (c-f). For instance, in rats (c), the NCCs migrate away from the neural crest before neurulation completes and while the neural fold is still open. In birds (d and f), neural crest cells do not migrate until the neural fold closes. In amphibians (e and f), the neural crest cells migrate after neurulation completes, and only after the cells have accumulated above the neural tube. Subsequently, NCCs will all migrate down their specialized pathways and diversify into the several sub-types of NCCs.

The hedgehog signaling pathway is a mechanism that directs the development of embryonic cells in animals, from invertebrates to vertebrates. The hedgehog signaling pathway is a system of genes and gene products, mostly proteins, that convert one kind of signal into another, called transduction. In 1980, Christiane Nusslein-Volhard and Eric F. Wieschaus, at the European Molecular Biology Laboratory in Heidelberg, Germany, identified several fruit fly (Drosophila melanogaster) genes. They found that when those genes were changed or mutated, the mutated genes disrupted the normal development of fruit fly larvae. The researchers called one of the genes hedgehog (abbreviated hh). Nusslein-Volhard, Wieschaus, and Edward B. Lewis, at the California Institute of Technology in Pasadena, California, shared the 1995 Nobel Prize for Physiology or Medicine for their research on how genes control early embryonic development in fruit flies. The hedgehog signaling pathway is conserved across many animal taxa or phyla, from Drosophila to humans. The hedgehog signaling pathway controls several key components of embryonic development, stem-cell maintenance, and it influences the development of some cancers.

Mesoderm is one of the three germ layers, groups of cells that interact early during the embryonic life of animals and from which organs and tissues form. As organs form, a process called organogenesis, mesoderm interacts with endoderm and ectoderm to give rise to the digestive tract, the heart and skeletal muscles, red blood cells, and the tubules of the kidneys, as well as a type of connective tissue called mesenchyme. All animals that have only one plane of symmetry through the body, called bilateral symmetry, form three germ layers. Animals that have only two germ layers develop open digestive cavities. In contrast, the evolutionary development of the mesoderm allowed in animals the formation of internal organs such as stomachs and intestines (viscera).

Neurocristopathies are a class of pathologies in vertebrates,
including humans, that result from abnormal expression, migration,
differentiation, or death of neural crest cells (NCCs) during embryonic development. NCCs are cells
derived from the embryonic cellular structure called the neural crest.
Abnormal NCCs can cause a neurocristopathy by chemically affecting the
development of the non-NCC tissues around them. They can also affect the
development of NCC tissues, causing defective migration or
proliferation of the NCCs. There are many neurocristopathies
that affect many different types of systems. Some neurocristopathies
result in albinism (piebaldism) and cleft palate in humans. Various
pigment, skin, thyroid, and hearing disorders, craniofacial and heart
abnormalities, malfunctions of the digestive tract, and tumors can be
classified as neurocristopathies. This classification ties a variety of
disorders to one embryonic origin.