Homologous structures not present in the adult organism often do appear in some stage of embryonic development. In this way, the embryo serves as a microcosm for evolution, passing through many of the stages of evolution to produce the current state of the organism. Species that bear little resemblance in their adult form may have strikingly similar embryonic stages.
For example, in humans, the embryo passes through a stage in which it has gill structures like those of the fish from which all terrestrial animals evolved. For a large portion of its development the human embryo also possesses a tail, much like those of our close primate relatives. This tail is usually reabsorbed before birth, but occasionally children are born with the ancestral structure intact. Tails and even gills could be considered homologous traits between humans and primates or humans and fish, even though they are not present in the adult organism.