When the mammalian oocyte is fertilized, a unit is formed which requires the collaboration of the parental cell genomes. In this collaboration, however, a fraction of the autosomal parental genes are provided with different values by a memory from the germ cell maturation process. In the article the importance of genomic imprinting by the parental cell genomes is discussed in terms of the regulation of embryonic development and with regard to tumourigenesis.