Many of the reactions and compounds involved in metabolism are almost iden tical in the different groups of living organisms. They are known as primary me tabolic reactions and primary metabolic products. In addition, however, a wide variety of biochemical pathways are characteristic of only a few species of organ isms, of single "chemical races," or even of a certain stage of differentiation of specialized cells. Such pathways are collectively referred to as "secondary metab olism," and the compounds formed are called "secondary products." Secondary products are frequently revealed by their...