Conditionally functional mutations are an important class of natural genetic variation, yet little is known about their prevalence in natural populations or how they mediate adaptive trajectories. In this talk I describe a vast reserve of cryptic genetic variation, alleles that are normally silent but which affect phenotype when the function of other genes is perturbed, in the gene networks of C. elegans embryogenesis. I find evidence that cryptic-effect loci are ubiquitous and segregate at intermediate frequencies in the wild. The cryptic alleles demonstrate low developmental pleiotropy, in that specific, rather than general, perturbations are required to reveal them. My findings underscore the importance of genetic background in characterizing gene function and provide a model for the expression of conditionally functional effects that may be fundamental in mechanisms of trait evolution.