Postembryonic Development

POSTEMBRYONIC DEVELOPMENT, development of the organism after birth or after hatching from the egg. In lower animals, postembryonic development is comparatively long and accompanied by very significant transformations. Since in this case the young animal differs greatly from the adult in its organization and lives in a different biological environment, and often in a different medium, it usually possesses a number of special adaptations or even temporary organs, which are later replaced by other definitive organs. Such postembryonic development is called larval. The transformation of the larva into the mature animal often proceeds very rapidly with disruption of the normal life functions of the organism. In this case, one speaks of metamorphosis. In most aquatic and especially marine animals, there are more or less clearly expressed larval forms. In coelenterates, they represent very simple two-layered bodies covered with a continuous ciliated epithelium ("planula"). In worms, larval forms are more differentiated: they usually possess an intestine divided into three sections, with oral and anal openings, a pair of primary excretory canals, a nerve center, and a primitive sensory apparatus, as well as organs of movement in the form of special ciliated threads encircling the larva ("trochophore" of annelid worms). The structure of mollusk and echinoderm larvae is somewhat more complex, and arthropod larvae are even more differentiated, possessing a segmented body and several pairs of swimming legs, a rather complex nervous system, and sensory organs. In arthropods, a sequential change of several characteristic larval forms, separated from each other by molting periods, is often observed. All these larvae float suspended in the water and feed also on microorganisms suspended in the water. In insects, the larval form is adapted to life on land, more often feeds on plant substances, and proves to be in its rather complex organization so well adapted to the specific conditions of its existence that the larval period of life often predominates over the period of maturity. Larval life sometimes continues for several months and even several years (in many beetles, in some cicadas). During this time, the larva grows strongly and accumulates reserve materials for the life of the adult animal (many insects in the adult state feed very little or not at all). Transformation into the adult animal occurs either gradually, i.e., in stages at each molt (incomplete metamorphosis), or suddenly during a relatively short time at the "pupa" stage, often under the protection of special coverings (cocoons), in the ground, etc. (complete metamorphosis). In the pupa, almost a complete restructuring of the entire organization of the animal occurs, the dissolution of larval organs, and the development of adult organs, mostly due to reserve embryonic cells (imaginal discs). In this case, a fully formed insect hatches from the pupa, which subsequently no longer grows. In vertebrates, fish fry hatch from the egg usually still far from perfect, differ from adults in their method of feeding and generally in their way of life, and often also possess specifically larval adaptations and organs (e.g., various appendages in pelagic forms). Of these, the suckers and external gills of the larvae of dipnoan and crossopterygian fishes are particularly interesting, as they are very similar to those of amphibian larvae. In the class of amphibians, the larval form reaches the highest (among vertebrates) stage of development, especially in tailless forms, in which a special body form ("tadpole") with a long tail, a special form of internal gills, special larval horny jaws, and a number of other features have developed, sharply distinguishing them from the adult form. This larval form passes into the final form by means of a relatively brief metamorphosis. With these few exceptions, postembryonic development of vertebrates usually proceeds according to the "direct" development type, in which the processes of morphogenesis occur with a certain gradualness and affect relatively few organs (especially the sex organs and secondary sexual characteristics), but the main development of the organism is expressed in its growth. Higher vertebrates are therefore born generally resembling adults, however, in their various groups, they are far from equally developed. Reptiles hatch from the egg already fully capable of independent existence. In birds, various ratios are observed depending on the presence and degree of care for the offspring—some (precocial birds) immediately run and are capable of feeding almost independently, others (altricial) are completely helpless and are fed by parents (sometimes with the help of the secretion of special crop glands). In mammals, care for the offspring has generally reached an even higher level, and although some forms are born already quite perfect and run freely (ungulates), they are still always fed for some time with the secretion of the mother's milk glands. The period of most rapid growth in mammals, as in other higher vertebrates, is the initial phases of the embryonic period. However, mammals grow very rapidly also during the lactation period. After switching to the normal food of the adult animal, growth usually immediately passes to a lower rate and subsequently regularly slows down until the onset of sexual maturity, when growth is often again sharply inhibited. After the onset of full sexual maturity, growth quite soon completely ceases. One should not think that postembryonic growth is proportional and that the body shape does not change during this time. On the contrary, due to the different rates of growth of various tissues, the growth of a differentiated organism is never proportional. This is manifested very clearly also in humans. The head and especially the brain grow much more slowly than the rest of the body and therefore have relatively the largest dimensions in the early stages of development. On the contrary, the trunk and limbs, especially the legs, grow faster than other parts and reach relatively the largest dimensions in the adult human. During the postembryonic period in mammals, the development of some organs also ends, such as kidneys, adrenal glands, almond-shaped glands, ossification of parts of the skeleton, eruption of teeth, and especially the development of secondary sexual characteristics. In humans, the largest postembryonic VESSELS

588 changes occur, first in early childhood in connection with the beginning of upright walking, and secondly in adolescents in connection with the onset of sexual maturity. Upright walking places the entire organism in new functional conditions. At this time, the final curvature of the spine is established, the pelvis and thoracic cage are finally formed (becomes flatter and wider); the shoulder blades move further to the dorsal side. Sexual maturity begins at the age of 11-13 years and is accompanied by the gradual development of a number of secondary sexual characteristics, which are finally formed several years later. In men, the shape of the larynx changes, the thoracic cage and musculature develop more strongly, body and facial hair (mustache, beard) develop. In women, the pelvis widens, there is a more significant deposition of subcutaneous fat, and mammary glands develop. After the onset of full sexual maturity, morphogenesis and growth can be considered generally completed. Postembryonic development can be accelerated and slowed down, can be correct (in the sense of maintaining the general proportionality of parts) and incorrect (in the sense of pathological deviations in the formation of the soma or psyche). Congenital diseases, for example syphilis, can significantly affect postembryonic development, causing a series of malformations of organs, sometimes lying at the basis of complex clinical-anatomical syndromes.