Scientists like Aleksandr Kovalevsky at the University of St. Germ Layer theory held that across all animal species, each of the germ layers gives rise to a fixed set of organs that are homologous between taxa. As a result of the renewed association of embryology and phylogeny, some argued that universal relationships of the germ layers may exist throughout the animal kingdom. Toward the end of the nineteenth century, interest in germ layers erupted. Huxley employed the term mesoderm for the middle germ layer in the 1871 edition of his A Manual of Anatomy of Vertebrated Animals. Those methods became foundational for the works of nineteenth century scientists like Charles Darwin, in England, and Ernst Haeckel, in Germany. Huxley's observation that developmental stages reflected evolution set a trend, in that scientists began to investigate evolutionary questions by studying embryos. When Huxley argued that the body architecture of the adult jellyfish was similar to those of vertebrate embryos, he united the vertebrate and invertebrate kingdoms and connected the study of growth and development, called ontogeny, with the study of relationships between organisms, called phylogeny. In 1849 natural historian Thomas Henry Huxley, in England, expanded the concept yet again, in his article "On the Anatomy and Affinities of the Family of the Medusae." Through his anatomical investigations of jellyfish, Huxley concluded that the two tissue layers he saw in the adult jellyfish bore the same relation to each other as the layers Pander had described in the chick embryo. Observations and Reflections), extended Pander's concept of the germ layers to apply to all vertebrates. Beobachtung und Reflexion ( On the Developmental History of Animals. Karl Ernst von Baer, professor of anatomy at the University of Königsberg, in Königsberg, Prussia, in his 1828 Über die Entwickelungsgeschichte der Thiere. Throughout his text, Pander wrote of both the independence and of the interdependence of these layers-while distinguishable, the layers appeared to work together to form organs.įollowing Pander's discovery, a series of nineteenth century scientists investigated the formation and derivatives of the germ layers. " Beiträge zur Entwickelungsgeschichte des Hühnchens im Eie" ( Contributions to the Developmental History of the Chicken in the Egg) described how two layers of the chick embryo give rise to a third, outlining the process of gastrulation in the chick, Gallus gallus. In 1817 Christian Pander received an MD from the University of Würzburg, in Würzburg, Germany, after completing his dissertation. Mesoderm, along with the other two germ layers, was discovered in the early nineteenth century. This patterning of the mesoderm organizes cells in specific locations along the dorso-ventral axis, and a cell's location determines what kinds of cell it and its daughter cells can become (cell fate). The process that gives rise to the mesoderm also creates a dorso-ventral pattern within the mesoderm. Mesendoderm has been found in species from Echinoderms, such as sea urchins, to mice, Mus musculus. Early in the reorganization process, a group of precursor cells that hold the potential to become either mesoderm or endoderm may form this tissue is called mesendoderm. During this process, the primary germ layers, endoderm and ectoderm, interact to form the third, called mesoderm. Gastrulation is an early stage of development during which an embryo, then a single-layered ball of cells called a blastula, reorganizes itself into a three-layered ball of cells, called a gastrula. In contrast, the evolutionary development of the mesoderm allowed in animals the formation of internal organs such as stomachs and intestines (viscera). Animals that have only two germ layers develop open digestive cavities. All animals that have only one plane of symmetry through the body, called bilateral symmetry, form three germ layers. 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. 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.
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