Animal embryos specify four early cell types. Determining the underlying mechanism is one central question of developmental biology. At present, it is studied how the crustacean Parhyale hawaiensis
manages such developmental program at the eight-cell stage. Specification of germ cells, e.g., depends on the genes vasa
, however, in a different way than in Drosophila melanogaster
, demonstrating change of gene function during evolution.