Variation in size and pallor of homozygous ζ^−/− null embryos produced in an outbred genetic background. The four 12-day pc embryos are the littermate progeny of a mating between ζ^−/− parents of the mixed 129/Black Swiss background. Note the variation in size among the littermates. Not easily discerned in the black and white photograph is the extreme pallor of the two embryos at the ends of the row.