Germ layer theory held that each of the germ layers, regardless of species, gave rise to a fixed set of organs. Kovalevsky's findings convinced many about the universality of the germ layers-a result that some scientists made a principle of germ layer theory. His research showed that invertebrate embryos had the same primary germ layers, endoderm and ectoderm, as vertebrate embryos, and that the layers arose in the same fashion across the animal kingdom. Petersburg, Russia, studied invertebrates. Beginning in 1864 embryologist Aleksandr Kovalevsky, who studied embryology at the University of St. These and other scientists began to look to embryos for evidence of evolution.īy the 1860s researchers compared germ layers across the animal kingdom. Huxley's support for a relationship between ontogeny and phylogeny, later known as the theory of recapitulation, would become fundamental to the works of late nineteenth century scientists, like Charles Darwin, in England, and Ernst Haeckel at the University of Jena, in Jena, Germany. The association that Huxley made between the body plan of the adult jellyfish and the vertebrate embryo connected the study of growth and development, called ontogeny, to the study of relationships between organisms, called phylogeny.

In his 1849 paper 'On the Anatomy and Affinities of the Family of the Medusae,' Huxley noted that the two layers of cells he saw in the adult jellyfish related to each other the same way as the germ layers in chick embryos described by Pander. Twenty years later, natural historian Thomas Henry Huxley, in England, applied Pander's concept of germ layers to jellyfish. Beobachtung und Reflexion ( On the Developmental History of the Animals.Observations and Reflections). Karl Ernst von Baer at the University of Königsberg, extended the concept of germ layers to include all vertebrates in his 1828 text Über die Entwickelungsgeschichte der Thiere. The germ layers drew the attention of many scientists in the nineteenth century. Rathke's finding marked the first discovery of endoderm and ectoderm in an invertebrate, but that information was not further investigated for two decades. Martin Rathke at the University of Königsberg, in Königsberg, Prussia (later Poland), soon found evidence in a developing crayfish, Astacus astacus, of the two layers Pander had described. Pander's description of the formation of these layers is the first account of gastrulation in the chick, and it grounded future studies of the germ layers. In his dissertation, Beiträge zur Entwickelungsgeschichte des Hühnchens im Eie ( Contributions to the Developmental History of the Chicken in the Egg), Pander described how two layers give rise to a third in the chick ( Gallus gallus) embryo. Within vertebrates, mesendoderm has been found in the zebrafish, Danio rerio, and has been indicated in mice, Mus musculus.Įndoderm, along with the other two germ layers, was discovered in 1817 by Christian Pander, a doctoral student at the University of Würzburg, in Würzburg, Germany. Scientists have found mesendoderm is widespread among invertebrates, including the nematode Caenorhabditis elegans, and the purple sea urchin, Strongylocentrotus purpuratus. Cells in the mesendoderm have the ability to differentiate into either mesoderm or endoderm, depending upon their position among surrounding cells. Throughout the early stages of gastrulation, a group of cells called mesendoderm expresses sets of both endoderm- and mesoderm-specific genes.

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

Ecto endo meso derm