Developmental stages for gametogenesis and early embryo development are expanded for clarity; those for later development are contracted, as indicated by double slashes. Note the very rapid changes in DNA methylation during: (i) gametogenesis - de novo methylation gives rise to substantially methylated genomes in the sperm and egg (albeit with differences in both the overall level of methylation and the pattern of methylation in these genomes - see text), and in (ii) the early embryo where a wave of genome-wide demethylation occurs at the preimplantation stage (morula and early blastula), and is succeeded shortly afterwards by large-scale de novo methylation beginning at the pregastrulation stage. The latter is particularly pronounced in somatic lineages, and to a lesser extent in trophoblast lineages giving rise to placenta and yolk sac, but does not occur in the primordial germ cells (the cells of the embryo which will eventually give rise to sperm and egg cells).