may be more explicitly canalized under genotypic control. Nucleotide substitution might still play a role here by modifying the level of activity rather than the specificity of neighboring loci, and elective recognition of transient states spontaneously derived then remains as a formal, if farfetched, possibility for other morphogenetic inductions. ReCerences and Notes 1. This definition excludes antibody-like substances such as the heniagglutinins found in normal human sera. These reagents do

more »

... ot, however, pose the problem of the mechanism of specific response which is the burden of this discussion., 2. Talmage , in this issue of Science, discusses various aspects of antibody specificity, including the number of antibodies, which may be exaggerated in current immunological thought. For the present discussion, however, this number is left open for experimental determination, for it would embarrass a theory of cellular selection only if it is large compared with the number of potential antibody-forming cells in the organism. To anticipate proposition Al, as few as five determinant amino acids would allow for 205 = 3,200,000 types of antibody.