The life cycle of a 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 air and water temperature. There are various stages of development of frogs, including the larval stage (egg - embryo - tadpole - frog). The metamorphosis of a tadpole into an adult is one of the most striking transformations in biology, as these changes prepare an aquatic organism for terrestrial existence.
Frog development: 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 and forelimbs, 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 toeating pond plants, disappear, the mouth and jaws take on a new shape, the muscles of the tongue develop more strongly, so that it is more convenient to catch flies and other insects. The elongated large intestine characteristic of herbivores shortens to accommodate the adult's carnivorous diet. At a certain stage in the development of frogs, the gills disappear, and the lungs increase.
What happens immediately after fertilization?
Shortly after fertilization, the egg begins to move from one cell stage to another in the process of division. The first cleavage starts at the animal pole and runs vertically down to the vegetative pole, dividing the egg into two blastomeres. The second cleavage occurs at right angles to the first, dividing the egg into 4 blastomeres. The third furrow is at right angles to the first two, being closer to the animal than to the vegetative pole. It separates the top four small pigmented areas from the bottom four. At this stage, the embryo already has 8 blastomeres.
Further splits become less regular. As a result, a unicellular egg gradually turns into a unicellular embryo, which at this stage is called a blastula, which, at the stage of 8-16 cells, begins to acquire spatial cavities filled with liquid. After a series of changes, a single-layer blastula turns into a two-layer embryo (gastrula). This complex process is called gastrulation. Intermediate stages of development of frogs at this stage implythe formation of three protective layers: ectoderm, mesoderm and endoderm, which are also known as primary germ layers. Later, the larvae hatch from these three layers.
Tadpoles (larval stage)
The next stage of frog development after the embryo is the larva, which leaves the protective shell already 2 weeks after fertilization. After the so-called release, the frog larvae 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 the 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 Metamorphoses
The aquatic tadpole undergoes a series of changes that eventually transform it into a frog. During metamorphosis, some larval structures are reduced and some are changed. Metamorphoses initiated by thyroid function can be divided into three categories.
1. Changes in appearance. Rear limbs grow, joints develop, fingers appear. The forelimbs, still hidden by special protective folds, come out. The tail shrinks, its structures break down and gradually nothing remains in its place. The eyes from the sides pass to the top of the head and become bulging, the system of the lateral line of the organs disappears, old skinis discarded, and a new one, with a large number of skin glands, develops. The horny jaws fall off along with the larval skin, they are replaced by true jaws, first cartilaginous and then bony. The gap in the mouth increases significantly, allowing the frog to feed on large insects.
2. Changes in the internal anatomy. The 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 blood circulation, more blood begins to flow into the lungs. The heart becomes three-chambered. The transition from a predominantly plant-based diet to a purely carnivorous diet affects the length of the alimentary canal. It shrinks and twists. The mouth becomes wider, the jaws develop, the tongue enlarges, the stomach and liver also become larger. Pronephros gives way to mesospheric buds.3. Changes in lifestyle. During the transition from larva to adult stage of development of frogs, with the onset of metamorphosis, the lifestyle of an amphibian changes. It rises to the surface more often to gulp air and inflate the lungs.
The frog is a miniature version of the adult frog
At the age of 12 weeks, the tadpole has only a small remnant of the tail and looks like a smaller version of the adult, which, as a rule, completes a full growth cycle by 16 weeks. The development and species of frogs are interrelated, some frogs that live at high altitudes or in cold places may live in the stagetadpole all winter. Certain species may have their own unique developmental stages that differ from the traditional ones.
The life cycle of a frog
Most frogs breed during the rainy season, when ponds are flooded with water. Tadpoles, whose diet differs from that of adults, can take advantage of the abundance of algae and vegetation in the water. The female lays eggs in a special protective jelly under water or on plants located nearby, and sometimes does not even care about the offspring. Initially, the embryos absorb their yolk reserves. Once the embryo has turned into a tadpole, the jelly dissolves and the tadpole emerges from its protective shell. The development of frogs from eggs to adults is accompanied by a number of complex changes (appearance of limbs, reduction of the tail, internal restructuring of organs, and so on). As a result, an adult individual of an animal in its structure, lifestyle and habitat differs significantly from previous stages of development.
