Gem layers is a basic term in embryology. They designate the layers of the body of the fetus at an early stage of its embryonic development. In most cases, these layers are epithelial in nature.
Gem layers are usually classified into three types:
• ectoderm - the outer sheet, which is also called the epiblast or skin-sensitive layer;
• Endoderm - inner layer of cells. It may also be called a hypoblast or entero-glandular;
• middle layer (mesoderm or mesoblast).
Germinal sheets (depending on their location, they are characterized by certain cell features. Thus, the outer layer of the embryo consists of light and tall cells, which are similar in structure to cylindrical epithelium. The inner leaf consists in most cases of large cells, which are filled with specific yolk lamellae and have a flattened appearance that makes them look like squamous epithelium.
Mesoderm at the first stage consists of spindle and stellate cells. They later form the epithelial layer. Needless to say, many researchers believe thatmesoderm is the middle germ layers, which are not an independent layer of cells.
The germ layers at first have the appearance of a hollow formation, which is called the blastodermal vesicle. At one of its poles, a group of cells gathers, which is 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 originates from the endoderm, and the skeleton, circulatory system and muscles originate from the mesoderm.
It should also be noted that special embryonic membranes are formed during embryogenesis. 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 embryos, which was first described by K. M. Baer in 1828. A little later, Charles Darwin identified the main reason for the similarity of embryos of all organisms - their common origin. Severov, on the other hand, argued that the common signs of embryos are associated with evolution, which proceeds in most cases through anabolism.
When comparing the main stages of development of embryos of different classes and animal species, certain features were found, which made it possible to formulate the law of embryonic similarity. The main provisions of thisthe 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 features that indicate its belonging to the corresponding genus and species. At the same time, the embryos of representatives of the same type are increasingly separated from each other, and their primary similarity is no longer traced.
