Postembryonic development

Embryonic period

In the embryonic period, three main stages are distinguished: crushing, gastrulation and primary organogenesis. The embryonic, or germinal, period of ontogenesis begins from the moment of fertilization and continues until the embryo emerges from the egg membranes. In most vertebrates, it includes stages (phases) of cleavage, gastrulation, histo- and organogenesis.

Splitting up

Main article : Cleavage (embryology)

Cleavage is a series of successive mitotic divisions of a fertilized or initiated egg. Cleavage is the first period of embryonic development, which is present in the ontogeny of all multicellular animals and leads to the formation of an embryo called blastula (one-layer embryo). At the same time, the mass of the embryo and its volume do not change, that is, they remain the same as those of the zygote, and the egg is divided into ever smaller cells – blastomeres. After each cleavage division, the cells of the embryo become smaller and smaller, that is, the nuclear-plasma relations change: the nucleus remains the same, and the volume of the cytoplasm decreases. The process proceeds until these indicators reach the values characteristic of somatic cells. The type of crushing depends on the amount of yolk and its location in the egg. If there is little yolk and it is evenly distributed in the cytoplasm (isolecithal eggs: echinoderms, flatworms, mammals), then crushing proceeds according to the type of complete uniform : blastomeres are the same in size, the entire egg is crushed. If the yolk is distributed unevenly (telolecital eggs: amphibians), then crushing proceeds according to the type of complete uneven : blastomeres are of different sizes, those that contain the yolk are larger, the egg is crushed as a whole. With incomplete crushing, there is so much yolk in the eggs that the crushing furrows cannot separate it entirely. Cleavage of an egg, in which only the “cap” of the cytoplasm concentrated on the animal pole, where the nucleus of the zygote is located, is crushed, is called incomplete discoidal (telolecital eggs: reptiles, birds). With incomplete surface crushing , the first synchronous nuclear divisions occur in the depths of the yolk, which are not accompanied by the formation of intercellular boundaries. The nuclei, surrounded by a small amount of cytoplasm, are evenly distributed in the yolk. When there are enough of them, they migrate to the cytoplasm, where, after the formation of intercellular boundaries, the blastoderm (centrolecithal eggs: insects) appears.


One of the mechanisms of gastrulation is invagination (protrusion of part of the blastula wall into the embryo) 1 – blastula, 2 – gastrula.

Gastrulation (invagination) – the gastrula is formed as a result of invagination of cells. During gastrulation, the cells of the embryo practically do not divide and do not grow. There is an active movement of cell masses (morphogenetic movements). As a result of gastrulation, germ layers (layers of cells) are formed. Gastrulation leads to the formation of an embryo called a gastrula.

Primary organogenesis

Primary organogenesis is the process of formation of a complex of axial organs. In different groups of animals, this process is characterized by its own characteristics. For example, in chordates at this stage, the laying of the neural tube, chord and intestinal tube occurs.

In the course of further development, the formation of the embryo is carried out due to the processes of growth, differentiation and morphogenesis. Growth ensures the accumulation of the cell mass of the embryo. During the process of differentiation, variously specialized cells arise, forming various tissues and organs. The process of morphogenesis ensures the acquisition of a specific form by the embryo.

Postembryonic development

Postembryonic development is either direct or indirect.

1. Direct development – development in which the emerging organism is identical in structure to an adult organism, but is smaller and does not have sexual maturity. Further development is associated with an increase in size and the acquisition of puberty. For example: the development of reptiles, birds, mammals.

2. Indirect development (larval development, development with metamorphosis) – the organism that has appeared differs in structure from the adult organism, is usually simpler, may have specific organs, such an embryo is called a larva. The larva feeds, grows, and over time, the larval organs are replaced by organs characteristic of an adult organism (imago). For example: the development of a frog, some insects, various worms.

Postembryonic development is accompanied by growth

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