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: Én 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.