The process of embryonic development of multicellular animals includes three main stages: cleavage , gastrulation and primary organogenesis . Embryog

Embryogenesis of animals (cleavage, gastrulation and organogenesis) [development]

The process of embryonic development of multicellular animals includes three main stages: cleavage, gastrulation and primary organogenesis.

Embryogenesis begins with the founding of the zygote (note: in parthenogenetic animals occurs not fertilization, but the ovum division activation).

Let’s take a look at the stages of embryonic development of multicellular animal on the marsh frog as an example (diagram 87).


Within a few hours (in other vertebrate species, even a few minutes) after the implementation sperm into the ovum the first stage of the embryogenesis begins — cleavage, which is a series of successive mitotic divisions of the zygote. At the same time with each division form increasingly smaller cells, called blastomeres (from Greek “blastos” — “germ”, “meros” — “part”). Cells shredding occurs by reducing the volume of the cytoplasm. Moreover, the process of cell division continues until the sizes of resulting cells are equal to the sizes of the other somatic cells of organisms of this species (the ovum — is one of the largest cells in the body). As a result, the mass of the embryo in the final period and its volume remain constant and approximately equal to the zygote.

Cleavage occurs this way. First, through zygote poles a dividing zygote extends dividing it in halves. Then the same way the second furrow passes, which divides the zygote on four blastomeres. Then, division is performed in the transverse and longitudinal directions. At the stage of 32 blastomeres, which is called the morula (from Latin “morum” — “mulberry”), embryo resembles a mulberry. Cleavage ends in about 128 stage blastomeres stage that form the blastula — a hollow sphere consisting of a single layer of cells.

Diagram 87. Stages of frog zygote cleavage: 1 — the beginning of the dividing furrow formation; 2 — the formation of the first two blastomeres; 3 — four blastomeres stage; 4 — eight blastomeres stage; 5 — 16 blastomeres stage (blastomeres start to vary the size); 6 — late morula (there are already tens of blastomeres with different sizes)
Diagram 88. The development of frog embryo after the passage of the cleavage process: 1 — blastula stage (only the primary body cavity); 2-6 — gastrula development stage, starting with a small incurvating and ending with the formation of a secondary cavity (diagrams 5 and 6 illustrate the exact same stage gastrula in different projections); 7-8 — primary organogenesis (neural plate is clearly visible at the top). Schemes presented in cut, with the exception of figure 7, where the external view of an embryo at the neurula stage is given (PC — primary cavity; SC — secondary cavity).


Once the blastula of the frog is fully formed, cells splitting is continued with particular intensity on one of the poles. This causes retraction of the part inward. As the result, a dual-layer embryo forms. This stage of ontogenesis is called gastrulation, and completes with formation of the gastrula (from Greek “gaster” — “stomach”). At this stage, the germ layers appear — layers of the embryo body of multicellular animals, which give rise to specific organs and tissues (diagram 88). In the most primitive multicellular animals — sponges and coelenterates — development completes at the stage of two germ layers (internal and external), and in the rest animals a third — middle layer — is formed. Taken from

Primary organogenesis

From the outer cell layer — the ectoderm (from Greek “ektos” — “outside” and “dermis” — “skin”) — tissues are formed, that perfor mainly an integumentary function. From outer cell layer, also part of the sense organs and of the digestive system are formed. From the inner layer of cells — the endoderm (from Greek “entos” — “inside” and “dermis”) — organs of the respiratory and digestive systems are formed, and derivatives of the intermediate layer — the mesoderm (from Greek “mezos” — “intermediate” and “dermis”) — operate a motor and a support function.

Thus, the process of primary organogenesis, which is also called neurulation (from Greek “neuron” — “nerve”), and the embryo at this stage — neurula, leads to the formation of organs, determining the base — structure of the body axis of the future adult. In vertebrates, in this period occurs the foundation of the chord, nervous and intestinal tubes. Then the rapid cell division begins, leading to a sharp increase in their number and consequent increase of the embryo. Thus cells differentiate, and various organs and tissues from them. The development of embryos of higher vertebrates (reptiles, birds and mammals) in the early stages of development is very similar to frog development. However, in them, starting from the stage of blastula, there is the appearance of specific embryonic membranes that protect the developing embryo from drying and other adverse environmental impacts. The presence of egg shells has made it possible for vertebrates to leave the aquatic habitat and become land creatures.

  • What are the two features that blastula is different from gastrula?

  • What biological systems form from the endoderm?