germ layers - a generic term in embryology.They represent the layers of the body of the fetus in the early stage of embryonic development.In most cases, these layers are epithelial character.
germ layers to classify in three types:
• ectoderm - the outer layer, which still bears the name of the epiblast, or skin-sensitive layer;
• endoderm - the inner layer of cells.It may also be called hypoblast or enteric-ferrous sheet;
• middle layer (mesoderm or mezoblast).
germ layers (depending on their location are characterized by certain features of the cells. Thus, the outer layer of the embryo is composed of bright and high-cells, which are structurally similar to columnar epithelium. The inner sheet is composed mostly of large cells that are filled with specific vitellineplates. They have a flattened appearance that makes them look like squamous epithelium.
mesoderm at the first stage consists of a spindle-shaped and stellate cells. They further form the epithelial layer. It must be said that many researchers believe that the mesoderm - is averagegerm layers that are not self-formation of cells.
first germ layers have the form of a hollow formation, which is called blastodermalnym bubble. At one of its poles is going to a group of cells, called cell mass. It gives rise to the primary colon (endoderm).
should be said that from the embryonic leaves form different organs.Thus, the nervous system arises from the ectoderm, the digestive tube begins from the endoderm, and the skeleton, circulatory system and muscles originate from the mesoderm.
It should also be noted that in embryogenesis, the Special embryonic membranes.They are temporary, do not participate in the formation of organs and exist only during embryonic development.Each class of living organisms can be traced certain features in the formation and structure of these shells.
With the development of embryology began to determine the similarity of embryos, which was first described by KMBaer in 1828.A little later, Charles Darwin identified the root cause of the similarity of embryos of all organisms - their common origin.Severs also argued that common symptoms are related to the evolution of the embryos, which occurs in most cases by anaboly.
When comparing the major stages of development of the embryos of different classes and types of animals have been found certain characteristics that allowed to formulate the law of embryonic similarity.The main provisions of this law was that the embryos of one type of organism in the early stages of its development are very similar.Subsequently, the embryo is characterized by more individual signs, which indicate that it belongs to the relevant genus and species.Thus the members of one type embryos more separated from each other, and their primary similarity has been observed.
