Germ leaves are the main term in embryology. They denote the layers of the fetal body at an early stage of its embryonic development. In most cases, these layers are epithelial in nature.
Germ leaves are usually classified into three types:
• ectoderm - the outer sheet, which is also called the epiblast or skin-sensitive layer;
• endoderm - the inner layer of cells. It can also be called a hypoblast or glandular sheet;
• the middle layer (mesoderm or mesoblast).
The embryonic leaflets (depending on their location, are characterized by certain cell characteristics. Thus, the outer layer of the embryo consists of light and tall cells, which in their structure are similar to a cylindrical epithelium. The inner leaf consists in most cases of large cells that are filled with specific yolk plates. They have a flattened appearance, which makes them look like a squamous epithelium.
The mesoderm in the first stage consists of spindle-shaped and stellate cells. They further form the epithelial layer. I must say that many researchers believe that the mesoderm is the middle germinal leaf, which is not an independent layer of cells.
The embryonic leaves first have the appearance of a hollow formation, which is called a blastodermal vesicle. At one of its poles, a group of cells is going to be called a cell mass. It gives rise to the primary gut (endoderm).
It should be said that different organs are formed from the embryonic leaves. Thus, the nervous system originates from the ectoderm, the digestive tube begins from the endoderm, and the skeleton, circulatory system and muscles come from the mesoderm.
It should also be noted that during embryogenesis special embryonic membranes are formed. They are temporary, do not participate in the formation of organs and exist only during embryonic development. Each class of living organisms has certain features in the formation and structure of these shells.
With the development of embryology, they began to determine the similarity of the embryos, which was first described by K.M. Baer in 1828. A little later, C. Darwin identified the main reason for the similarity of the embryos of all organisms - their common origin. Severov argued that common signs of embryos are associated with evolution, which in most cases proceeds through anabolism.
When comparing the main stages of development of embryos of different classes and species of animals, certain features were discovered, which made it possible to formulate the law of embryonic similarity. The main provisions of this law was that the embryos of organisms of the same type in the early stages of their development are very similar. Subsequently, the embryo is characterized by more and more individual traits that indicate its belonging to the corresponding genus and species. In this case, the embryos of representatives of one type are more and more separated from each other, and their primary similarity is no longer traced.