To talk about gastrulation, let's take as an example what happens in chordate animals, represented by amphioxo and frogs.
Chordates are animals that have notochord, a flexible rod that sits on the back of the embryo. The notochord persists in the adult of some chordate animals, such as amphioxus. In vertebrate animals, excluding some fish, the notochord regresses totally or almost totally and the spine develops from the mesoderm.
O amphioxus It is an animal about 6 cm long that lives buried in the sand in shallow waters of the marine environment, leaving only the anterior region of the body. These animals have separate sexes and fertilization is external.
O amphioxus egg is oligolithite and your segmentation is total subequal. Gastrulation occurs by a process called invagination of blastomers into the blastocele, as if a finger pushed the wall of a bladder. The blastocele shrinks and even disappears. At the point of invagination arises a hole called blastopore; the internal cavity that forms is the primitive or archenectal gut.
In gastrulation, the germinal or embryonic leaflets, which will give rise to all tissues and organs. These leaflets are: ectoderm (the most external), mesoderm (the intermediary) and endoderm (the most internal).
Animals that have three germ leaflets are called triblastics or triploblastics., as is the case with chordates. There are, however, animals that have only two germinal leaflets: ectoderm and endoderm. These animals are called diblastics or diploblastics, as is the case of the cnidarians.
The above scheme simply describes amphioxus gastrulation. In this case, the inner layer that directly covers the archenter is called the mesentoderm and will give rise immediately after the mesoderm and endoderm. (Some consider mesentoderm as endoderm and mesoderm formed from endoderm.)
Nas frogs The fertilization is external, eggs are heteroliths and segmentation is totally unequal. The eggs have a gelatinous wrap that dries out on contact with air. Thus all embryonic development takes place in water. An aquatic larva is formed, the tadpole that undergoes metamorphosis, originating the adult. In these cases, we talk about indirect development, because there is a larval phase. When the larval phase is not present, one speaks of direct development.
Frog gastrulation occurs by invagination and also by epibolia. By invagination a slit forms: the blastopore. By epibolia the micromers begin to divide rapidly and eventually cover the macromers. Micromeres are primarily insinuated by the ventral lip. The blastopore takes on the appearance of a circle. The micromers creep into the blastocele, delimiting the archenter. There is also the differentiation of the three germ leaflets: the ectoderm, the mesoderm and the endoderm.
Thus, in frog gastrulation, in addition to the embryo increasing in volume, three other characteristics are fundamental:
- formation of embryonic or germinal leaflets, which will give rise to all tissues and organs;
- formation of the archenterus or primitive bowel;
- blastopore formation, orifice of communication of the archenter with the exterior.