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What is the beginning and end of the embryonic period of development. Embryonic period




1. Where does the mammalian embryo develop?

The fertilized egg enters the uterus, where the process of its fixation and development takes place.

2. What is a zygote?

A zygote is a single-celled stage in the development of an organism that occurs as a result of the fusion of a spermatozoon and an egg.

Questions

1. How does the embryonic period of development begin and end?

The embryonic period (embryogenesis) begins at the moment of zygote formation and ends with birth (for example, in mammals) or exit from the egg membranes (for example, in birds).

2. How does the postembryonic period of development begin and end?

The postembryonic period begins from the moment of birth or the release of the organism from the egg membranes and lasts until the end of the life of the individual.

3. What development is called direct? Give examples of animals with direct development.

Postembryonic development is called direct, when a creature similar to an adult (reptiles, birds, mammals) appears from an egg or mother's body.

4. What development is called indirect? Give examples of animals with indirect development.

Postembryonic development is called indirect, when the larva formed in the embryonic period is simpler than the adult organism, and differs from it in the ways of feeding, movement, etc. (intestinal, flat and annelids, crustaceans, insects, amphibians).

5. What is the Müller-Haeckel biogenetic law?

The individual development of an individual (ontogeny) to a certain extent repeats the historical development of the species (phylogenesis) to which the given individual belongs.

6. What is the meaning of the biogenetic law?

The biogenetic law is very important, since it testifies to the common ancestors of animals belonging to different systematic groups. It allows the use of embryological data to reconstruct the course of phylogenesis.

Tasks

1. Using the knowledge gained in the study of the "Animals" section, give examples of postembryonic development associated with a change in lifestyle and habitat.

In a frog, for example, a larva (tadpole) develops from an egg, which differs from adult animals in structure, lifestyle and habitat. The tadpole, like fish, has gills, a lateral line organ, a tail, a two-chambered heart, and one circle of blood circulation. The larva feeds, grows and eventually turns into a frog.

The presence of the larval stage in the development of amphibians and many other animals provides them with the opportunity to live in different environments and use different food sources. Thus, the tadpole lives in water and feeds on plant food, while the frog leads mainly a terrestrial lifestyle and feeds on animal food.

A change in habitat and, as a result, a change in the way of life of an animal during its transition from the larval stage to an adult organism reduces intraspecific competition. In addition, in some sedentary or attached animals (coral polyps, oysters, mussels, etc.), the free-swimming larva contributes to the spread of the species and the expansion of its range. This avoids overpopulation, which would lead to increased competition for food and other resources that would threaten the survival of the species.

2. In the early period of development, the heart of the human embryo consists of one atrium and one ventricle. Based on the provisions of the biogenetic law, comment on this fact.

The human embryo in the process of individual development repeats the past stage of evolution (a two-chambered heart is a sign of fish).

The process of ontogenesis begins with the embryonic period of development (embryogenesis), which lasts from the formation of a zygote to birth, or exit from the egg. In this period, according to a certain scheme, there is an active growth and development of the embryo.

Splitting up

The first stage of embryogenesis is called cleavage. Its feature is the rapid division of cells, in which there is practically no growth. The number of cells increases, but their size decreases.

Rice. 1. Cleavage and types of blastula.

This stage begins with the division of the zygote into two cells. Then each of the resulting cells, called blastomeres, also divides. It turns out 4, 8, 16, 32, etc. blastomeres.

As a result, a multicellular embryo is formed, which has the shape of a bubble. This stage is called blastula and contains several thousand cells.

gastrulation

The second stage of embryogenesis is gastrulation. It begins with the invagination of the blastula wall inward. In this case, a gastrula is formed, which has a two-layer wall. The outer layer of cells is called the ectoderm and the inner layer is called the endoderm.

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Rice. 2. Gastrulation in the lancelet.

Subsequently, a third layer appears between the ectoderm and endoderm - the mesoderm. These layers are called germ layers.

At the stage of gastrulation, the synthesis of proteins characteristic of a particular cell type begins. This starts the process of differentiation - the growth of differences between parts of the embryo.

With the differentiation of cells from each germ layer, certain tissues and organ systems are subsequently formed. For example, from the mesoderm - muscle tissue, and from the ectoderm - skin epithelium.

Organogenesis

Gastrulation ends with the beginning of the formation of the first organs:

  • neural tube;
  • chords;
  • intestines.

In all groups of animals, the same organs are formed from the same germ layers. This is proof of the unity of the animal world.

Rice. 3. Formation of axial organs in the lancelet.

Primary organogenesis is characterized by the phenomenon of embryonic induction - the mutual influence of developing organs on each other.

When the organism is ready for life in an open atmosphere, it is released from the embryonic membranes, which ends the embryonic period of development.

Critical periods

In some periods of embryogenesis, the embryo is especially sensitive to the influence of environmental factors.

These periods are called critical. These include, for example, the period of 20-24 weeks of pregnancy in a woman, when the main functional systems of the unborn child are formed.

Exposure to stress, infection, injury, etc. during the critical period is more harmful to the fetus than at other times.

What have we learned?

Embryogenesis is the development of an organism before birth. It consists of three stages. At the stage of crushing, the number of cells increases. At the stage of gastrulation, the cells differentiate and three germ layers are formed. Organs appear at the stage of organogenesis.

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The proembryonic period preceding the formation of the zygote is associated with the formation of gametes. Otherwise, it is gametogenesis (ovogenesis and spermatogenesis).

The processes that characterize oogenesis lead to the formation of a haploid set of chromosomes and the formation of complex structures in the cytoplasm. r-RNA and m-RNA accumulate in the egg, the yolk accumulates . The type of development of ontogeny depends on the amount of yolk and its distribution.

Depending on the amount of yolk, 4 types of eggs are distinguished:

1) polylecithal(a lot of yolk; lecitos - gr. yolk);

2) mesolecithal(average amount of yolk);

3) oligolecithal(small amount of yolk);

4) alecithal(the yolk is almost absent).

Rice. 1. Types of oocytes according to the distribution of yolk: a - alecithal, b - isolecital, c - telolecital, d - centrolecital.

According to the distribution of the yolk, the egg cells are divided into 3 types:

1) isolecithal or homolecithal(with a uniform distribution of the yolk over the egg), by the amount of yolk they are more often oligo- or alecithal. Examples: eggs of echinoderms, lower chordates, mammals.

2) telolecithal(the yolk is concentrated at the vegetative pole; Greek end); According to the content of the yolk, these eggs are most often poly- or mesolecithal. Examples: eggs of mollusks, fish, amphibians, reptiles, birds.

3) centrolecithal(the yolk is concentrated in the center of the cell, and the cytoplasm along the periphery and inside the nucleus). Examples: Insect eggs, according to the yolk content, these are most often oligo- or mesolecithal eggs.

1.5. Embryonic period

The embryonic period (Greek embryon - embryo) begins with fertilization and the formation of a zygote. The end of this period in different types of ontogeny is associated with different moments of development.

Embryonic period is divided into the following stages:

1) fertilization - the formation of a zygote;

2) crushing - the formation of blastula;

3) gastrulation - the formation of germ layers;

4) histo- and organogenesis - the formation of organs and tissues of the embryo.

In the larval form of ontogenesis, the embryonic period begins with the formation of a zygote and ends with the exit from the egg membranes.

In the non-larval form of ontogenesis, the embryonic period begins with the formation of a zygote and ends with the exit from the embryonic membranes.

With the intrauterine form of ontogenesis, the embryonic period begins with the formation of a zygote and lasts until birth.

1.5.1. Zygote.

A zygote is a single-celled stage in the development of a new organism. In the zygote, the stage of two pronuclei and the stage of synkaryon are distinguished. The two pronucleus stage precedes the synkaryon stage. The sperm has entered the egg, but the nuclei of the sperm and egg have not yet fused. The stage of the synkaryon is characterized by the fusion of nuclei. As a result of syncaryogamy, the diploid set of chromosomes is restored. After the formation of the synkaryon, the zygote proceeds to fragmentation.

Fig.2. Fertilization in mammals. A - the sperm enters the egg; B - the nucleus was formed from the head of the spermatozoon, and the centriole was formed from the neck. 1 - egg nucleus, 2 - spermatozoon, 3 - receptive tubercle, 4 - centriole, 5 - sperm nucleus.

Animal ontogeny

Comparison of vertebrate embryos at different stages of embryonic development. The infamous illustration from the work of Ernst Haeckel, in which the differences between embryos are artificially low, in order to better comply with the theory of recapitulation (the repetition of phylogenesis in ontogenesis). It should be noted that the falsification of this illustration does not negate the fact that embryos usually really seem to be more similar to each other than adult organisms, which was noted by embryologists even before the emergence of the theory of evolution.

Ontogeny is divided into two periods:

  1. embryonic - from the formation of a zygote to birth or exit from the egg membranes;
  2. postembryonic - from the exit from the egg membranes or birth to the death of the organism.

Embryonic period

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

Splitting up

Cleavage - a series of successive mitotic divisions of a fertilized or initiated egg for development. 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 full uniform: blastomeres are the same in size, the whole egg is crushed. If the yolk is distributed unevenly (telolecital eggs: amphibians), then crushing proceeds according to the type 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 at the animal pole, where the nucleus of the zygote is located, is crushed, is called incomplete discoidal(telolecithal eggs: reptiles, birds). At incomplete surface crushing in the depths of the yolk, the first synchronous nuclear divisions occur, 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 then, after the formation of intercellular boundaries, the blastoderm (centrolecithal eggs: insects) appears.

gastrulation

One of the mechanisms of gastrulation is invagination (the invagination of part of the blastula wall into the embryo) 1 - blastula, 2 - 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, the laying of the neural tube, notochord, and intestinal tube occurs at this stage.

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

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