A change in the form or function of a living
   organism, by a natural process of growth or development; as, the
   metamorphosis of the yolk into the embryo, of a tadpole into a frog, or
   of a bud into a blossom. Especially, that form of sexual reproduction
   in which an embryo undergoes a series of marked changes of external
   form, as the chrysalis stage, pupa stage, etc., in insects. In these
   intermediate stages sexual reproduction is usually impossible, but they
   ultimately pass into final and sexually developed forms, from the union
   of which organisms are produced which pass through the same cycle of
   changes. See Transformation.