Metamorphosis: In biology, 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 behavior. From animal development.
Metamorphosis, the transformation of the larva into an adult, is a more or less complicated process depending on the degree of difference between the two forms. The transformation may be gradual, extend over a long period, and involve a number of intermediate stages; alternatively, the transformation may be achieved in one step. In the latter case, especially if the difference between the larva and adult is great, large parts of the body of the larva, including all the specifically larval organs, disintegrate (necrobiotic metamorphosis).
Critical mass: ?n nuclear physics, the minimum amount of a given fissile material necessary to achieve a self-sustaining fission chain reaction under stated conditions. Its size depends on several factors, including the kind of fissile material used, its concentration and purity, and the composition and geometry of the surrounding reaction system. History & Philosophy. The study either of the historical process and its development or of the methods used by historians to understand their material. The term history may be employed in two quite different senses: it may mean (1) the events and actions that together make up the human past, or (2) the accounts given of that past and the modes of investigation whereby they are arrived at or constructed.