The biogenetic law is a theory of development and evolution proposed by Ernst Haeckel in Germany in the 1860s. It is one of several recapitulation theories, which posit that the stages of development for an animal embryo are the same as other animals' adult stages or forms. Commonly stated as ontogeny recapitulates phylogeny, the biogenetic law theorizes that the stages an animal embryo undergoes during development are a chronological replay of that species' past evolutionary forms. The biogenetic law states that each embryo's developmental stage represents an adult form of an evolutionary ancestor. According to the law, by studying the stages of embryological development, one is, in effect, studying the history and diversification of life on Earth. The biogenetic law implied that researchers could study evolutionary relationships between taxa by comparing the developmental stages of embryos for organisms from those taxa. Furthermore, the evidence from embryology supported the theory that all of species on Earth share a common ancestor.

Gordon Watkins Douglas researched cervical cancer, breach delivery, and treatment of high blood pressure during pregnancy in the US during the twentieth century. He worked primarily at Bellevue Hospital Center in New York, New York. While at Bellevue, he worked with William E. Studdiford to develop treatments for women who contracted infections as a result of illegal abortions performed throughout the US in unsterile environments. Douglas also established the first contraception and pregnancy termination clinic at Bellevue Hospital shortly after the legalization of abortion as a result of the 1973 US Supreme Court ruling in Roe v. Wade. Furthermore, Douglas showed that fetal and maternal cells exchange between the pregnant woman and fetus during pregnancy, which led to the later development of non-invasive prenatal testing in the early twenty-first century.

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.

In 1828, while working at the University of Konigsberg in Konigsberg, Germany Karl Ernst von Baer proposed four laws of animal development, which came to be called von Baer's laws of embryology. With these laws, von Baer described the development (ontogeny) of animal embryos while also critiquing popular theories of animal development at the time. Von Baer's laws of embryology provided a framework to research the relationships and patterns between the development of different classes of organisms, and the patterns between this development and the diversification of species on Earth (phylogeny).

Franz Keibel studied the embryos of humans and other animals in Europe at the turn of the twentieth century. He lived and worked in several different parts of Germany and France. Keibel drew illustrations of embryos in many stages of development. Keibel used these illustrations, which he and others in the scientific community called normal plates, to describe the development of organisms in several species of vertebrates. His illustrations are published in the sixteen-volume text Normentafeln zur Entwicklungsgeschichte der Wirbelthiere (Normal Plates of the Developmental history of Vertebrates), published in 1895, and in the Manual of Human Embryology, which he edited with Franklin Paine Mall of the US, published in 1912. Keibel's plates showed human embryos in different stages of development between the twelfth day and the second month after fertilization.

In 2006, Kazutoshi Takahashi and Shinya Yamanaka reprogrammed mice fibroblast cells, which can produce only other fibroblast cells, to become pluripotent stem cells, which have the capacity to produce many different types of cells. Takahashi and Yamanaka also experimented with human cell cultures in 2007. Each worked at Kyoto University in Kyoto, Japan. They called the pluripotent stem cells that they produced induced pluripotent stem cells (iPSCs) because they had induced the adult cells, called differentiated cells, to become pluripotent stem cells through genetic manipulation. Yamanaka received the Nobel Prize in Physiology or Medicine in 2012, along with John Gurdon, as their work showed scientists how to reprogram mature cells to become pluripotent. Takahashi and Yamanaka's 2006 and 2007 experiments showed that scientists can prompt adult body cells to dedifferentiate, or lose specialized characteristics, and behave similarly to embryonic stem cells (ESCs).

In 1978, James Kitching discovered two dinosaur embryos in a road-cut talus at Roodraai (Red Bend) in Golden Gate Highlands National Park, South Africa. Kitching assigned the fossilized embryos to the species of long necked herbivores Massospondylus carinatus (longer vertebra) from the Early Jurassic period, between 200 and 183 million years ago. The embryos were partially visible but surrounded by eggshell and rock, called matrix. Kitching said that the eggs were too delicate to remove from the matrix without damage. Twenty-seven years later in 2005, Diane Scott, a member of a team led by Robert Reisz from the University of Toronto in Toronto, Canada, uncovered the two almost complete, well-articulated embryos. Scientists have inferred information from the embryos about Massospondylus dinosaurs' growth, development, and behaviors including parental care, gait, and locomotion.

Ilya Ilyich Mechnikov studied phagocytes, immune function, and starfish embryos in Europe during the late nineteenth and early twentieth centuries. Mechnikov adopted the French form of his name, Élie Metchnikoff, in the last twenty-five years of his life. In 1908, he won the Nobel Prize in Physiology or Medicine with Paul Ehrlich for their contributions to immunology. Mechnikov discovered phagocytes, immune cells that protect organisms by ingesting foreign particles or microorganisms, by conducting experiments on starfish larvae. He then developed a theory of the cellular process involving phagocytes, known as phagocytosis, to explain how inflammation is a part of the self defense system found in both vertebrates and invertebrates. His experimental work was part of the tradition of evolutionary embryology, which emerged in the decades following the publication of Charles Darwin's On the Origin of Species in 1859, and was influenced by Ernst Haeckel's concept of the biogenetic law.

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.

The endothelium is the layer of cells lining the blood vessels in animals. It weighs more than one kilogram in adult humans, and it covers a surface area of 4000 to 7000 square meters. The endothelium is the cellular interface between the circulating blood and underlying tissue. As the medium between these two sets of tissues, endothelium is part of many normal and disease processes throughout the body. The endothelium responds to signals from its surrounding environment to help regulate functions like the resistance that blood vessels need to pump blood through the body (vasomotor tone), the policing of substances trying to enter or exit the blood vessel (blood vessel permeability), and the ability of blood to clot (hemostasis). In addition to diseases like atherosclerosis, endothelium has been indicated as a component in pathologies like cancer, asthma, diabetes, hepatitis, multiple sclerosis, and sepsis. The shape, size, and appearance of endothelial cells, called their phenotypes, vary depending upon which part of the body the cells are from, a property called phenotypic heterogeneity. The endothelium, its properties, and its responses to stimuli are governed largely by the local environment of the cells.