Gastrula is the stage that the embryo of a multicellular animal goes through during its development. The blastula is transformed into gastrul. This is the previous stage of embryo development. The process of formation and growth of gastrula is called gastrulation. Then comes the stage of neurula.
The structure of the embryo during this period
As you know, gastrula cells form the so-called petals. They correspond to three layers. The external is called exoderm, and in the future it turns into the epidermis - the nails, hair and nervous system of the adult body.
The middle lobe of the gastrula is called the mesoderm. Muscles, skeleton, endocrine and circulatory systems grow out of it. But not all living organisms have a middle layer of cells. Some simple invertebrate animals develop from a two-layer gastrula.
The endoderm is the inner layer of the embryo. From it, the lungs, liver and intestines form. The human fetus also has a gastrula stage. It is formed in a disk-like form already on the 8th-9th day of fertilization. But, nevertheless, it is gastrul, as in amphibians with reptiles.
Methods of gastrulation
Modern biology knows several of them:
- Intussusception. Occurs in intestinal and even higher animals. Scyphoid jellyfish and corals in the embryo phase develop precisely by the method of invagination. This method leads to the retraction of the wall inward, and the formation of a hole, which in the future often becomes the mouth of the primary creatures, and the second mouth - the anus or cesspool. Primary and small animals are simple animals. Some are not even visible to the human eye. These are arthropods, mollusks, nematodes, annelid worms, tardigrades, etc. Higher creatures include echinoderms and chordates. Including a person.
- Immigration. It means that the cells enter inside the blastula and form a special important tissue called parenchyma from the inside. It is usually observed in sponges and intestinal cavities, on the example of which the great Russian scientist I.I. Mechnikov established that gastrula is not a simple stage of the embryo, but an unusual discovery in world embryology.
- Delamination. Translated from Latin as "layered". This method of gastrulation is possible due to the splitting of the blastula cells into two layers, from which ectoderm and endoderm later form. Such a simple type of organogenesis is inherent in higher mammals.
- Epibolism. In some fish and amphibians, gastrula develops in this way. In this case, small, yolk-poor cells grow around one large, in which there is enough yolk. The result is gastrula, similar in composition to a bird's egg.
These four methods of gastrulation are rarely found in nature in its purest form. More often their combinations are observed.
Name history
The Russian biologist G. Kovalevsky in 1865 believed that gastrula is an "intestinal larva", due to the similarity of gastrula to the larva and its location in the area close to the intestine. After less than one decade, in 1874, the German philosopher and natural scientist E. Haeckel introduced the term "gastrula", which is translated from ancient Greek as "womb", "stomach", which is also explained by the location of the embryo.
Independent organism
As a rule, gastrula is an embryo that does not exist by itself. It is located in the egg or uterus. But in nature there are also animals that develop from free-floating gastrul. Most often, it is intestinal. This group of creatures is interesting for its simple structure, which in an adult is similar to the composition of gastrula. It follows from this that she is the same independent organism as the animal, which eventually grows out of it. It can perform all the functions necessary to maintain vital activity in the embryonic state.