Metamorphosis

METAMORPHOSIS (from Greek meta- after and morphe- appearance, form, kind), a change in the appearance and structure of an animal in post-embryonic development until it reaches the mature state; it is called otherwise transformation or indirect development (metabolism). In animals with metamorphosis, a larva emerges from the egg, either underdeveloped compared to the adult (for example, in the bedbug) or of a completely different structure than the adult animal (for example, flea larvae, trochophore of marine annelids). The transformation of the larva into an adult animal occurs either gradually (for example, the transformation in the frog, newt) or is associated with the passage of stages that are sharply different both in external appearance and in structure. In animals whose growth is associated with molting (i.e., the periodic shedding of the old skin, in particular of a chitinous nature, for example, crustaceans, insects, etc.), the various stages of metamorphosis are sharply delimited from each other both in form and in the time of appearance. From the egg of Anopheles a larva emerges; three times it molts in the water, increasing in size; after the third molt a pupa emerges from the larva, and from the pupa a winged mosquito hatches. The larva, pupa, and imago are sharply different from each other in this case. Metamorphosis in some species proceeds shortened, in other cases, on the contrary, it is complicated by the addition of new stages (hypermetamorphosis). In some animal species (mollusks, echinoderms), whose eggs have a large amount of nutrient yolk, development is direct, i.e., without transformation, whereas species with eggs poor in nutrient yolk undergo metamorphosis.

E. Pavlovsky. Mechanism of metamorphosis. Metamorphosis of amphibians is studied in most detail. During the transformation of a tadpole into a frog, the majority of organs undergo a substantial reorganization: the hind legs grow first, and then the fore legs, the tail is resorbed, the gills are absorbed, the structure of the jaws and the whole skull changes, the intestine shortens 6–7 times, the chemistry of intestinal digestion changes, the circulatory apparatus is reorganized, the integuments change, etc. The process of metamorphosis can proceed without the participation of the central nervous system, which is seen from the experiments of Lebe and Wintriber, where after significant damage to the central nervous system metamorphosis was not disturbed. Transplantation experiments of eyes (Ulenhut), gills (Kornfeld), and intestines (Sembrat) from a young larva of an amphibian to an old one led to the synchrony of the transformation of the tissues of the host and the transplanted organ. This testifies to the participation of humoral influences in the processes of metamorphosis. At the same time, in a number of cases, the autonomous behavior of the transplant (for example, skin) has been established. A piece of skin of a beginning to transform axolotl, being transplanted to a non-transforming axolotl, undergoes metamorphosis going to the end. The source of humoral influences on metamorphosis is apparently the hormone of the thyroid gland. Feeding with thyroid gland substance leads to accelerated transformation of the tadpole into a frog (Gudernatsch, Romais, etc.) and to the transformation of a neotenic amphibian, the axolotl, into the terrestrial stage (amblystoma) (see Axolotl, Neoteny). Thyroidectomy performed on tadpoles by Allen, Goskins, Swingle, and Schultze fully confirmed the position on the participation of the thyroid gland, since thyroidectomized larvae did not undergo transformation. Metamorphosis of amphibians can be stimulated both by thyroid gland preparations, synthetic thyroxin, and other iodine-containing compounds (diiodotyrosine, iodglobulin, etc.) and even by crystalline iodine. Removal of the glandular part of the pituitary gland in amphibian larvae prevents the onset of metamorphosis, apparently in connection with the atrophy of the thyroid gland. Hypotheses have been expressed that the connection between the parts of the transforming tadpole organism is reduced to the interconditionality of resorption and proliferation processes, simultaneously occurring during metamorphosis. According to Champy, the hormone of the thyroid gland stimulates the process of cell division, and the proliferating tissues already secondarily cause disintegration in the areas undergoing resorption. Along with this, the opposite hypothesis has been expressed. The increased activity of the thyroid gland during metamorphosis should rather stimulate disintegration processes in places of least resistance. In turn, the resorption of tissues of the intestine, gills, and tail can contribute to the multiplication of cells of neighboring parts (lungs, tongue, fore limbs, etc.) through the mediation of decomposition products and mitogenetic rays arising in the foci of resorption. A number of studies have shown that resorbing organs (gills, intestine, tail) are indeed sources of mitogenetic rays during metamorphosis. In addition, it was shown that when mitogenetic rays act on the fore limb of a tadpole through a quartz plate, the latter increases in size. Mitogenetic radiation in resorbing tissues is also detected during metamorphosis of tail-bearing amphibians (axolotls, newts). The source of mitogenetic radiation is also the blood, and during the height of metamorphosis the radiation of the blood of the tadpole or axolotl intensifies many times, which in turn can be connected with proliferative phenomena in individual tissues. Physiological changes in the process of metamorphosis of amphibians and insects were studied, and it was established in amphibians that nitrogen excretion sharply intensifies during the height of resorption processes; this coincides with the phenomenon of intensified gas exchange. Proteolytic enzymes of the digestive tract are activated before the beginning of metamorphosis, and then their activity sharply falls. Essential changes also occur in the amount of nitrogen-containing components of the blood. In insects during metamorphosis, namely in the pupal stage, the content of protein decomposition products sharply increases, which is connected with intensive histolysis of larval tissues. At the same time, the tissues of the insect in the end of the larval and the beginning of the pupal stage become a source of mitogenetic radiation. The influence of thyroid gland preparations on metamorphosis of insects was in most cases not possible to establish. Metamorphosis of insects is apparently connected with the activity of the nervous system, since removal of the supraesophageal ganglion led to the absence of transformation. L. Blekhore. In pathology, metamorphosis denotes processes of tissue change of quite different, often opposite character. Thus, one speaks of progressive metamorphosis, having in mind the transformation of tissue of one kind into another, for example, deciduous metamorphosis of the mucous membrane of the uterus during pregnancy, cavernous metamorphosis of a thrombus, when the organization of the latter is accompanied not only by massive development of vessels in it, but also by their expansion. Even more often one speaks of regressive metamorphosis. This includes various types of transformations, necrobiotic changes of tissues arising for the most diverse reasons; in connection with this, one speaks, for example, of fatty metamorphosis, mucous metamorphosis, etc. However, regressive nature should be understood here conditionally, since the masses of detritus and various decomposition products arising during tissue disintegration play often a very significant role in the process of progressive metamorphosis. The concept of progressive metamorphosis in anatomy borders on the idea of so-called metaplasia (see).