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The primitive cell layers, or first tissues, which appear early in the development of animals and from which the embryo body and its auxiliary membranes, when present, are constructed. These are more or less distinct anatomically, but do not necessarily have sharp boundaries of demarcation. Germ layers are almost universal among animal embryos and appear to establish discontinuities of architectural importance without complete loss of continuity. Three kinds of germ layers are recognizable: (1) the ectoderm or outer skin, (2) the endoderm or inner skin, and (3) the mesoderm or middle skin. The layers have been named in accordance with their positions in the spherical type of gastrula such as that of the sea urchin or amphibian. The terms epiblast, mesoblast, and hypoblast are sometimes used as synonyms for ectoderm, mesoderm, and endoderm, respectively. The three primary germ layers are present as a basic structural plan in all Metazoa with the exception of the coelenterates and the Porifera, in which a distinct mesodermal layer is absent. See Gastrulation
layers of the body of the embryo of multi-cellular animals and man, which are formed in the process of gastrulation.
The majority of organisms have three germ layers: the exterior, or ectoderm; the interior, or endoderm; and the middle, or mesoderm. Exceptions are the sponges and coelenterates, in which only two germ layers are formed, the exterior and the interior. Derivatives of the ectoderm perform integumental, sensory, and motor functions: in the course of embryonic development they give rise to the nervous system; the skin integument and the skin glands that are formed from it; the hair, plumage, scales, nails, and so forth; the epithelium of the anterior and posterior sections of the digestive system; the connective-tissue foundation of the skin; the pigment cells; and the visceral skeleton. The endoderm forms the lining of the intestinal cavity and provides nutrition for the embryo; from it originate the mucous membrane of the digestive system, the digestive glands, and the organs of respiration. The mesoderm effects communication between the parts of the embryo and performs supportive and trophic functions; from it are formed the organs of excretion, the sex organs, the circulatory system, and the serous membranes that line the secondary body cavity (coelom) and clothe the internal organs and muscles; in vertebrates the skeleton is also formed from the mesoderm. Analogous germ layers in different groups of organisms may have, in addition to common traits, substantial differences in their manner of formation and their structures, because of the adaptation of the embryos to different conditions of development.
The science of germ layers—one of the most important generalizations in embryology—has a long history and is connected with fundamental biological teachings and discoveries, such as epigenesis, cell theory, and Darwinism. Among those responsible for its development were K. F. Wolff, C. H. Pander, K. E. Baer, R. Remak, E. Haeckel, O. Hertwig, A. O. Kovalevskii, and I. I. Mechnikov. The latest data of experimental embryology, including the results of in vivo staining of various sections of the wall of the blastula and tracing of their migration during gastrulation and neurulation, have made it possible to determine, as early as the blastula stage, the positions of groups of cells from which the various germ layers and their derivatives will be formed and to construct a map of the future rudiments of organs and their systems. Through experiments in transplanting and removing the material of various germ layers in the blastula stage and during the period of gastrulation, the properties of the matter of various germ layers and their capacity for differentiation have been elaborated: at the beginning, the sections of the various germ layers can still be substituted for one another when transplanted, but by the end of gastrulation they lose that capacity.
T. A. DETLAF