Ontogeny is the course of development of an organism from fertilized egg to adult; phylogeny is the evolutionary history of a group of organisms. So the phrase, originating in 19th-century biology, means that as an embryo of an advanced organism grows, it will pass through stages that look very much like the adult phase of less advanced organisms. For example, at one point the human embryo has gills and resembles a tadpole.
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