(Metamorphosis) in biology, (refers to a) striking change of form or structure in an individual after hatching or birth. Hormones called molting and juvenile hormones, which are not species specific, apparently regulate the changes. These physical changes as well as those involving growth and differentiation are accompanied by alterations of the organism's physiology, biochemistry, and behaviour.

The immature forms, or larvae, are adapted to environments and modes of life that differ from those of the adult forms. These differences may be of significance in assuring that larvae and adults of the same species do not engage in direct competition for food or living space. Examples of metamorphosis include the tadpole, an aquatic larval stage that transforms into the land-dwelling frog (class Amphibia). Starfishes and other echinoderms undergo a metamorphosis that includes a change from the bilateral symmetry of the larva to the radial symmetry of the adult. Metamorphic patterns are well-known in crabs, lobsters, and other crustaceans and also in snails, clams, and other mollusks. The larval form of the urochordate (e.g., the tunicate, or sea squirt) is tadpole-like and free swimming; the adult is sessile and somewhat degenerate.

Among the most dramatic and thoroughly studied examples of metamorphosis are the insects. Because development is not the same in all insects, it is convenient to group them into major categories according to the pattern of structural changes:

1. Ametabolous- In ametabolous development there is simply a gradual increase in the size of young until adult dimensions are attained. This kind of development occurs in the silverfish, springtail, and other primitive insects.
   
2. Hemimetabolous- In more advanced insects (e.g., grasshoppers, termites, true bugs) a phenomenon known as gradual, or hemimetabolous, metamorphosis occurs. The hemimetabolous life cycle consists of egg, nymph, and adult. The nymph, or immature insect, resembles the adult in form and eating habits, differing in size, body proportions, and colour pattern. Rudimentary wings are visible and develop externally. Development is gradual through a series of molts (periodic shedding of the outer skeleton), the adult emerging from the final molt.
   
3. Holometabolous- Complete, or holometabolous, metamorphosis is characteristic of beetles, butterflies and moths, flies, and wasps. Their life cycle includes four stages: egg, larva (q.v.), pupa (q.v.), and adult. The larva differs greatly from the adult. It is wingless, and its form and habits are suited for growth and development rather than reproduction. The change to the adult occurs during the inactive, nonfeeding pupal stage. At this time the larva undergoes a transformation in which the wings appear externally, larval organs and tissues are broken down, and adult structures are developed.

Hypermetamorphosis, a form of complete metamorphosis, occurs in some beetles, flies, and other insects and is characterized by a series of larval stages.

...Another offshoot of the carbon experiments reached fruition sooner. Samuel Langley, Henry Draper, and other American scientists needed a highly sensitive instrument that could be used to measure minute temperature changes in heat emitted from the Sun's corona during a solar eclipse along the Rocky Mountains on July 29, 1878. To satisfy those needs Edison devised a “microtasimeter” employing a carbon button. This was a time when great advances were being made in electric arc lighting, and during the expedition, which Edison accompanied, the men discussed the practicality of “subdividing” the intense arc lights so that electricity could be used for lighting in the same fashion as with small, individual gas “burners.” The basic problem seemed to be to keep the burner, or bulb, from being consumed by preventing it from overheating. Edison thought he would be able to solve this by fashioning a microtasimeter-like device to control the current. He boldly announced that he would invent a safe, mild, and inexpensive electric light that would replace the gaslight.

The incandescent electric light had been the despair of inventors for 50 years, but Edison's past achievements commanded respect for his boastful prophecy. Thus, a syndicate of leading financiers, including J.P. Morgan and the Vanderbilts, established the Edison Electric Light Company and advanced him $30,000 for research and development. Edison proposed to connect his lights in a parallel circuit by subdividing the current, so that, unlike arc lights, which were connected in a series circuit, the failure of one lightbulb would not cause a whole circuit to fail. Some eminent scientists predicted that such a circuit could never be feasible, but their findings were based on systems of lamps with low resistance—the only successful type of electric light at the time. Edison, however, determined that a bulb with high resistance would serve his purpose, and he began searching for a suitable one.

He had the assistance of 26-year-old Francis Upton, a graduate of Princeton University with an M.A. in science. Upton, who joined the laboratory force in December 1878, provided the mathematical and theoretical expertise that Edison himself lacked. (Edison later revealed, “At the time I experimented on the incandescent lamp I did not understand Ohm's law.” On another occasion he said, “I do not depend on figures at all. I try an experiment and reason out the result, somehow, by methods which I could not explain.”)

By the summer of 1879 Edison and Upton had made enough progress on a generator—which, by reverse action, could be employed as a motor—that Edison, beset by failed incandescent lamp experiments, considered offering a system of electric distribution for power, not light. By October Edison and his staff had achieved encouraging results with a complex, regulator-controlled vacuum bulb with a platinum filament, but the cost of the platinum would have made the incandescent light impractical. While experimenting with an insulator for the platinum wire, they discovered that, in the greatly improved vacuum they were now obtaining through advances made in the vacuum pump, carbon could be maintained for some time without elaborate regulatory apparatus. Advancing on the work of Joseph Wilson Swan, an English physicist, Edison found that a carbon filament provided a good light with the concomitant high resistance required for subdivision. Steady progress ensued from the first breakthrough in mid-October until the initial demonstration for the backers of the Edison Electric Light Company on December 3.

It was, nevertheless, not until the summer of 1880 that Edison determined that carbonized bamboo fibre made a satisfactory material for the filament, although the world's first operative lighting system had been installed on the steamship Columbia in April. The first commercial land-based “isolated” (single-building) incandescent system was placed in the New York printing firm of Hinds and Ketcham in January 1881. In the fall a temporary, demonstration central power system was installed at the Holborn Viaduct in London, in conjunction with an exhibition at the Crystal Palace. Edison himself supervised the laying of the mains and installation of the world's first permanent, commercial central power system in lower Manhattan, which became operative in September 1882. Although the early systems were plagued by problems and many years passed before incandescent lighting powered by electricity from central stations made significant inroads into gas lighting, isolated lighting plants for such enterprises as hotels, theatres, and stores flourished—as did Edison's reputation as the world's greatest inventor.

One of the accidental discoveries made in the Menlo Park laboratory during the development of the incandescent light anticipated the British physicist J.J. Thomson's discovery of the electron 15 years later. In 1881–82 William J. Hammer, a young engineer in charge of testing the light globes, noted a blue glow around the positive pole in a vacuum bulb and a blackening of the wire and the bulb at the negative pole. This phenomenon was first called “Hammer's phantom shadow,” but when Edison patented the bulb in 1883 it became known as the “Edison effect.” Scientists later determined that this effect was explained by the thermionic emission of electrons from the hot to the cold electrode, and it became the basis of the electron tube and laid the foundation for the electronics industry.

Edison had moved his operations from Menlo Park to New York City when work commenced on the Manhattan power system. Increasingly, the Menlo Park property was used only as a summer home. In August 1884 Edison's wife, Mary, suffering from deteriorating health and subject to periods of mental derangement, died there of “congestion of the brain,” apparently a tumour or hemorrhage. Her death and the move from Menlo Park roughly mark the halfway point of Edison's life.

(Homology) in biology, (refers to a) similarity of the structure, physiology, or development of different species of organisms based upon their descent from a common evolutionary ancestor. Homology is contrasted with analogy, which is a functional similarity of structure based not upon common evolutionary origins but upon mere similarity of use. Thus the forelimbs of such widely differing mammals as humans, bats, and deer are homologous; the form of construction and the number of bones in these varying limbs are practically identical, and represent adaptive modifications of the forelimb structure of their common early mammalian ancestors. Analogous structures, on the other hand, can be represented by the wings of birds and of insects; the structures are used for flight in both types of organisms, but they have no common ancestral origin at the beginning of their evolutionary development. A 19th-century British biologist, Sir Richard Owen, was the first to define both homology and analogy in precise terms.

When two or more organs or structures are basically similar to each other in construction but are modified to perform different functions, they are said to be serially homologous. An example of this is a bat's wing and a whale's flipper. Both originated in the forelimbs of early mammalian ancestors, but they have undergone different evolutionary modification to perform the radically different tasks of flying and swimming, respectively. Sometimes it is unclear whether similarities in structure in different organisms are analogous or homologous. An example of this is the wings of bats and birds. These structures are homologous in that they are in both cases modifications of the forelimb bone structure of early reptiles. But birds' wings differ from those of bats in the number of digits and in having feathers for flight while bats have none. And most importantly, the power of flight arose independently in these two different classes of vertebrates; in birds while they were evolving from early reptiles, and in bats after their mammalian ancestors had already completely differentiated from reptiles. Thus, the wings of bats and birds can be viewed as analogous rather than homologous upon a more rigorous scrutiny of their morphological differences and evolutionary origins.

A dentition with different kinds of teeth (heterodonty)—incisors, canines, and cheek teeth—is characteristic of all primates and indeed of mammals generally. Heterodonty is a primitive characteristic, and primates have evolved less far from the original pattern than most mammals. The principal changes are a reduction in the number of teeth and an elaboration of the cusp pattern of the molars.

The dental formula of primitive placental mammals is assumed to have been 5 . 1 . 4 . 3 / 5 . 1 . 4 . 3 = 44 teeth (the numbers being the numbers respectively of pairs of incisors, canines, premolars, and molars in the upper and lower jaws). No living primate has retained more than two incisors in the upper jaw. The incisors are subject to considerable variation in strepsirrhines. Characteristically, the upper incisors are peglike, one or the other pair often being absent; in the lower jaw, the incisors show a peculiar conformation that has been likened structurally and functionally to a comb. This dental comb is composed of the lower canines and lower incisors compressed from side to side and slanted forward; the most specialized dental combs—seen, for example, in the fork-crowned lemur (genus Phaner) and the needle-clawed galago (genus Euoticus)—are used for scraping exudates off bark, but other species use the structure for piercing fruit, for nipping off leaves, and for grooming the fur. Canines are present throughout the order but show remarkable variation in size, shape, projection, and function. Characteristically, the teeth of Old World monkeys have a function in the maintenance of social order within the group as well as an overtly offensive role; their function as organs of digestion is relatively unimportant. They are large and subject to sexual dimorphism, being larger in males than females. Great apes have smaller canines than Old World monkeys, though still sexually dimorphic; human canines are smaller still, and there is no size difference between the sexes.

The trend in the evolution of the cheek teeth has been to increase the number of cusps and reduce the number of teeth. Both molars and premolars show this tendency. No living primate has four premolars; primitive primates, tarsiers, and New World monkeys have retained three on each side of each jaw, but in the apes and Old World monkeys, there are only two premolars. The primitive premolars are uniform in shape and are unicuspid, but in primates the most posterior premolar tends to evolve either one or two extra cusps (molarization), an adaptation that extends the cheek-tooth row for a herbivorous diet. In species with large upper canines, the most anterior lower premolar assumes a peculiar shape known as sectorial, functioning as a hone for the scythelike canine. In humans, whose canines are small and unremarkable, the first and second premolars are identical in shape and two-cusped.

The trend in the morphology of the molars has been to increase the primitive three cusps to four or five, the less-insectivorous species having four cusps on the molar crown in the upper jaw and five cusps on the lower. A tendency in smaller New World monkeys has been to reduce the molar series from three to two in both jaws.