Embryonic leaflets: their types and structural features

Germinal leaves are the main term in embryology. They designate the layers of the fetal body at an early stage of its embryonic development. In most cases, these layers are epithelial.

The embryonic leaflets are classified into three types:

• ectoderm - the outer leaf, which is still called epiblast or skin-sensitive layer;

• endoderm - the inner layer of cells. It can also be called a hypoblast or an enteric glandular sheet;

• middle layer (mesoderm or mesoblast).

The embryo leaves (depending on their location, are characterized by specific features of the cells: for example, the outer layer of the embryo consists of light and tall cells, which in their structure are similar to the 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 flat epithelium.

The mesoderm in the first stage consists of spindle-shaped and stellate cells. They subsequently form an epithelial layer. I must say that many researchers believe that the mesoderm is a medium embryonic leaf that is not an independent layer of cells.

The embryonic sheets first have the form of a hollow formation, which is called a blastodermal vesicle. At one of its poles a group of cells is assembled, which is called the cell mass. It gives rise to the primary gut (endoderm).

It should be said that different organs are formed from embryonic sheets. Thus, the nervous system arises from the ectoderm, the digestive tube starts from the endoderm, and the skeleton, the circulatory system, and the muscles come from the mesoderm.

It should also be noted that in 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 traces 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 K.M. Baer in the year 1828. A little later, Charles Darwin identified the main reason for the similarity of embryos of all organisms - their common origin. Severov also argued that the common signs of embryos are associated with evolution, which occurs in most cases through anabolism.

When comparing the main stages of development of embryos of different classes and species of animals, certain features were found that allowed us to formulate the law of embryonic similarity. The main provisions of this law are that the germs of organisms of one type in the early stages of their development are very similar. Subsequently, the embryo is characterized by increasingly individual traits that indicate its belonging to the corresponding genus and species. In this case, embryos of representatives of one type are increasingly separated from each other, and their primary similarity is no longer traceable.