A phenomenon seen in the early embryonic development of members of various animal groups, including certain nematodes, crustaceans, insects, and vertebrates, in which some of the genetic material is eliminated from progenitor body (somatic) cells during cell division. In some cases the somatic chromosomes break down, with only a few remaining intact, whereas the integrity of chromosomes in the germ-cell lineage is maintained. It serves to silence or reduce the dosage of certain genes and is thought to have evolved independently in different lineages of organisms.