The life cycle and stages of development of frogs

The life cycle of the frog, gametogenesis, fertilization and other seasonal activities depend on numerous external factors. The life of almost all amphibians depends on the number of plants and insects in the pond, as well as the temperature of the air and water. Different stages of development of frogs, including larval (egg-embryo-tadpole-frog), stand out. The metamorphosis of a tadpole into an adult is one of the most striking transformations in biology, since these changes prepare the aquatic organism for terrestrial existence.

Development of frogs: photo

In tailless amphibians, such as frogs and toads, metamorphic changes are most pronounced, almost every organ undergoes modification. The shape of the body changes beyond recognition. After the appearance of the hind limbs and front, the tail gradually disappears. The cartilaginous skull of the tadpole is replaced by the facial skull of the young frog. The horny teeth that the tadpole used to eat pond plants disappear, the mouth and jaws take a new shape, the muscles of the tongue develop more strongly to make it easier to catch flies and other insects. The elongated colon, characteristic of herbivores, is shortened to adapt to the carnivorous diet of an adult. At a certain stage in the development of frogs, the gills disappear, and the lungs increase.

What happens immediately after fertilization?

Soon after fertilization, the egg begins to move from one cell stage to another in the process of division. The first cleavage starts from the animal pole and runs vertically down to the vegetative pole, dividing the egg into two blastomeres. The second splitting occurs at right angles to the first, dividing the egg into 4 blastomeres. The third furrow is located at right angles to the first two, being closer to the animal than to the vegetative pole. It separates the four upper small pigmented sections from the four lower ones. At this stage, the embryo already has 8 blastomeres. Further splits become less regular. As a result, the unicellular egg is gradually transformed into a unicellular embryo, which at this stage is called a blastula, which at the stage of 8-16 cells begins to acquire fluid-filled spatial cavities. After a number of changes, a single-layer blastula turns into a two-layered embryo (gastrula). This complex process is called gastrulation. Intermediate stages of development of frogs at this stage involve the formation of three protective layers: ectoderm, mesoderm and endoderm, which are also known as primary embryonic leaves. Later, larvae hatch from these three layers.

Tadpoles (larval stage)

The next stage in the development of the frog after the embryo is the larva, which leaves the protective shell only 2 weeks after fertilization. After the so-called liberation, the larvae of frogs are called tadpoles, which are more like small fish about 5-7 mm long. The body of the larva includes a distinct head, trunk and tail. The role of respiratory organs is played by two pairs of small external gills. A fully formed tadpole has organs adapted for swimming and breathing, the lungs of the future frog develop from the pharynx.

Unique metamorphosis

The water tadpole undergoes a number of changes, which eventually transform it into a frog. During metamorphosis, some larval structures are reduced, and some change. Metamorphoses initiated by the function of the thyroid gland can be divided into three categories.

1. Changes in appearance. Growing hind legs, joints develop, fingers appear. Front limbs, still hidden by special protective folds, come out. The tail decreases, its structures break down and gradually nothing remains in its place. Eyes from the sides go to the top of the head and become bulging, the system of the lateral line of the organs disappears, the old skin is discarded, and the new one, with a large number of skin glands, develops. Horny jaws fall away together with the larval skin, they are replaced by the true jaws, first cartilaginous, and then bony. The rupture of the mouth increases significantly, allowing the frog to feed on large insects.

2. Changes in internal anatomy. Gills begin to lose their importance and disappear, the lungs become more and more functional. Corresponding changes occur in the vascular system. Now the gills gradually cease to play a role in the circulation, more blood begins to enter the lungs. The heart becomes three-chambered. The transition from a predominantly plant food to a purely carnivorous diet affects the length of the digestive canal. He shrinks and twists. The mouth becomes wider, the jaw develops, the tongue increases, the stomach and liver also become larger. Pronefros is replaced by mesospheric kidneys.

3. Changes in lifestyle. During the transition from the larva to the adult stage of development of frogs with the onset of metamorphosis the way of life of the amphibian changes. It often rises to the surface to swallow air and inflate the lungs.

Froggy - a miniature version of an adult frog

At the age of 12 weeks, the tadpole has only a small tail residue and looks like a smaller version of the adult, which by the 16th week usually completes a full growth cycle. The development and species of frogs are interrelated, some frogs that live at high altitudes or in cold places can live in a tadpole all winter. Certain species can have their own unique stages of development, which differ from traditional ones.

The life cycle of a frog

Most frogs multiply in the rainy season, when water is flooded with water. Tadpoles, whose diet is different from eating adults, can benefit from an abundance of algae and vegetation in the water. Eggs in a special protective jelly the female lays under the water or on plants located nearby, and sometimes does not even care about the offspring. Initially, embryos absorb their yolk stocks. As soon as the embryo has turned into a tadpole, the jelly dissolves and the tadpole emerges from its protective shell. The development of frogs from caviar to an adult is accompanied by a number of complex changes (the appearance of limbs, tail reduction, internal organ rearrangement, and so on). As a result, the adult animal in its structure, lifestyle and habitat significantly differs from previous stages of development.