Embryo Project Encyclopedia Articles

Conrad Hal Waddington's "Experiments on Embryonic Induction III," published in 1934 in the Journal of Experimental Biology, describes the discovery that the primitive streak induces the mammalian embryo. Waddington's hypothesis was that a transplanted primitive streak could induce neural tissue in the ectoderm of the rabbit embryo. The primitive streak defines the axis of an embryo and is capable of inducing the differentiation of various tissues in a developing embryo during gastrulation. In this experiment Waddington was, in fact, able to induce neural differentiation. Waddington noted that the tissue is "competent"; for a chick organizer, and by deduction a mammalian organizer must exist. Competence refers to a cell's ability to respond to an inducing signal, which is temporally limited to certain developmental stages. Waddington's initial work laid the foundation for many decades of research to follow, including further experiments by Waddington with the mammalian organizer.

Rosalind Elsie Franklin worked with X-ray crystallography at King's College London, UK, and she helped determine the helical structure of DNA in the early 1950s. Franklin's research helped establish molecular genetics, a field that investigates how heredity works on the molecular level. The discovery of the structure of DNA also made future research possible into the molecular basis of embryonic development, genetic disorders, and gene manipulation.

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.

The Cell-Theory was written by Thomas Henry Huxley in Britain and published in 1853 by The British and Foreign Medico-Chirurgical Review. The twenty-two page article reviews twelve works on cell theory, including those in Germany by Caspar Friedrich Wolff in the eighteenth century and by Karl Ernst von Baer in the nineteenth century. Huxley spends much of The Cell-Theory on a cell theory proposed in the late 1830s by Matthias Schleiden and Theodor Schwann in Germany. Schleiden and Schwann maintained that the cell was the most fundamental unit of life and that the nucleus was the most significant cellular component. Huxley, instead, promoted an epigenetic theory of the cell, for which properties of life emerge from the outer cytoplasm, cell membrane, and wall (the periplast), as opposed to the inner contents of the cell, including the nucleus (the endoplast). Huxley's arguments in The Cell-Theory influenced future scientists about the role of epigenetic processes in embryology and development.

Frederik Ruysch, working in the Netherlands, introduced the term epithelia in the third volume of his Thesaurus Anatomicus in 1703. Ruysch created the term from the Greek epi, which means on top of, and thele, which means nipple, to describe the type of tissue he found when dissecting the lip of a cadaver. In the mid nineteenth century, anatomist Albrecht von Haller adopted the word epithelium, designating Ruysch's original terminology as the plural version. In modern science, epithelium is a type of animal tissue in which cells are packed into neatly arranged sheets. The epithelial cells lie proximate to each other and attach to a thin, fibrous sheet called a basement membrane. Epithelia line the surfaces of cavities and structures throughout the body, and also form glands. Although they lack blood vessels, epithelia contain nerves and can function to receive sensation, absorb, protect, and secrete, depending on which part of the body the epithelia line. During development, epithelia act in conjunction with another tissue type, mesenchyme, to form nearly every organ in the body, from hair and teeth to the digestive tract. Epithelia are an essential part of embryonic development and the maintenance and function of the body throughout life.

Hilde Proscholdt Mangold was a doctoral student at the Zoological Institute at the University of Freiburg in Freiburg, Germany, from 1920-1923. Mangold conducted research for her dissertation 'On the Induction of Embryonic Primordia by Implantation of Organizers from Different Species' ('Ueber Induktion von Embryonanlagen durch Implantation artfremder Organisatoren'), under the guidance of Hans Spemann, a professor of zoology at the University of Freiburg. The dissertation was the culmination of five experiments on three species of newt embryos, of the genus Triton (presently, Triturus), performed during the summers of 1921 and 1922, which resulted in a confirmation of Spemann's organizer concept. Spemann and Mangold published the dissertation in a 1924 edition of Roux's Archives for Microscopic Anatomy and Developmental Mechanics (Roux's Archiv fur Mikroskopische Anatomie und Entwicklungsmechanik)."

Calvin Blackman Bridges studied chromosomes and heredity in the US throughout the early twentieth century. Bridges performed research with Thomas Hunt Morgan at Columbia University in New York City, New York, and at the California Institute of Technology in Pasadena, California. Bridges and Morgan studied heredity in Drosophila, the common fruit fly. Throughout the early twentieth century, researchers were gathering evidence that genes, or what Gregor Mendel had called the factors that control heredity, are located on chromosomes. At Columbia, Morgan disputed the theory, but in 1916, Calvin Bridges published evidence that, according to Morgan, did much to convince skeptics of that theory. Bridges also established that specific chromosomes function in determining sex in Drosophila.

Alfred Henry Sturtevant studied heredity in fruit flies in the US throughout the twentieth century. From 1910 to 1928, Sturtevant worked in Thomas Hunt Morgan’s research lab in New York City, New York. Sturtevant, Morgan, and other researchers established that chromosomes play a role in the inheritance of traits. In 1913, as an undergraduate, Sturtevant created one of the earliest genetic maps of a fruit fly chromosome, which showed the relative positions of genes along the chromosome. At the California Institute of Technology in Pasadena, California, he later created one of the first fate maps, which tracks embryonic cells throughout their development into an adult organism. Sturtevant’s contributions helped scientists explain genetic and cellular processes that affect early organismal development.

From 1913 to 1916, Calvin Bridges performed experiments that indicated genes are found on chromosomes. His experiments were a part of his doctoral thesis advised by Thomas Hunt Morgan in New York, New York. In his experiments, Bridges studied Drosophila, the common fruit fly, and by doing so showed that a process called nondisjunction caused chromosomes, under some circumstances, to fail to separate when forming sperm and egg cells. Nondisjunction, as described by Bridges, caused sperm or egg cells to contain abnormal amounts of chromosomes. In some cases, that caused the offspring produced by the sperm or eggs to display traits that they would typically not have. His research on nondisjunction provided evidence that chromosomes carry genetic traits, including those that determine the sex of an organism.

In 1910, Thomas Hunt Morgan performed an experiment at Columbia University, in New York City, New York, that helped identify the role chromosomes play in heredity. That year, Morgan was breeding Drosophila, or fruit flies. After observing thousands of fruit fly offspring with red eyes, he obtained one that had white eyes. Morgan began breeding the white-eyed mutant fly and found that in one generation of flies, the trait was only present in males. Through more breeding analysis, Morgan found that the genetic factor controlling eye color in the flies was on the same chromosome that determined sex. That result indicated that eye color and sex were both tied to chromosomes and helped Morgan and colleagues establish that chromosomes carry the genes that allow offspring to inherit traits from their parents.