Mammals have even fewer cubs, and many species have only one cub. Caring for them often lasts for a year or more (wolf). In ontogenesis, the development of an individual, there are three periods: embryonic (development in the egg or the mother's body), postembryonic (from the moment of release from the egg or birth to full maturation) and the period of adulthood. In some species, development is direct: the body, starting from a fertilized female egg, develops in a certain direction, increasing the number of cells, becoming more complex and increasing in size to an adult state, usually determined by the ability to reproduce.


In other animals, development proceeds in leaps and bounds through different stages, during which the animal radically changes its external and internal characteristics and the very way of existence. An example of this development is insects with a full cycle of transformations. A caterpillar develops from a butterfly egg, it has tenacious short legs, a powerful gnawing mouth, and a huge intestine. Everything is adapted to eating large amounts of green food and rapid growth. But a caterpillar makes a butterfly. It is much smaller, with wings, long limbs, and a tube-like proboscis adapted to feeding on flower nectar. For such a complex and profound change, the caterpillar first turns into a pupa, an almost immobile creature with a large supply of nutrients. A butterfly is "formed" inside the shell of the pupa. Amphibians develop with transformations among vertebrates. The tadpole of a frog has a respiratory and circulatory system, arranged like a fish, a mouth and an entire digestive system adapted to life in a body of water. In the process of metamorphosis, an aquatic animal — a tadpole — turns into a terrestrial frog. The ontogenesis of each animal is closely related and, to a certain extent, predetermined by the phylogeny of this species. In 1866, the German scientist E. Haeckel formulated a biogenetic law: ontogenesis repeats phylogeny, i.e. the development of an individual is a brief rapid repetition of the main stages of phylogenesis. The biogenetic law was subsequently developed by A. N. Severtsov and other Russian and foreign scientists. The validity of the biogenetic law can be seen in the example of human development. The phylogeny of higher animals began with unicellular animals — human life begins with an egg. Primitive bilayer multicellular animals resemble a bilayer embryo — gastrula. Cartilaginous fish develop gill slits — they are on the sides of the neck and in the human embryo. When amphibians come ashore, the gill slits disappear, and they are not preserved in the human embryo, the last, closest human ancestor was a monkey covered with fur. The skin of a mature human fetus is covered with germ fuzz, i.e., a fairly thick hairline. However, in ontogenesis there is no strict sequence of repetition of the stages of development of a species in the development of an individual. In the above example, the human embryo does not repeat the adult stages of fish, amphibians, and reptiles, but is similar only to their embryos. Limited-time promotions require quick action, as the code may stop working after a certain date or number of redemptions. A smart bettor who finds the code online should register immediately rather than waiting for a better opportunity. The 1win bonus code today limited-time promotion is available only while supplies last, and missing the window means losing access to the 500% boost. Once registered with the code, the bonus is locked in even if the promotion ends later for new users. Acting quickly ensures you do not miss this chance to multiply your first deposit fivefold.