Answer
Ontogeny is the growth of any multicellular living thing from fertilized egg to adult form.
Phylogeny is the evolutionary history of a group.
A famous phrase “ontogeny recapitulates phylogeny” by German biologist Ernst Haeckel in 1866 was for many decades accepted as natural law (the biogenetic law). It means that an organism, in the course of its development, goes through all the stages of those forms of life from which it has evolved. For example, if all vertebrates evolved from fish, then in their early development all vertebrate embryos should resemble fish. If the next stage in the evolution of mammals was amphibian, all mammal embryos should next resemble amphibians. This continues until the adult form of each animal is reached.
However, modern biology now rejects this dogmatic perspective. Though recognizing that human beings evolved from fish and reptiles, biologists cannot discern in our development any stages that correspond to those of a fish or a reptile.
Work Step by Step
Modern biology states that related organisms in the earliest stages of development often differ greatly, but there is a stage (zootype or phylotypic stage) at which all vertebrate embryos do bear a general resemblance to each other, and then eventually diverge into distinct adult morphologies as they complete their development. This gives us the embryiological similarities seen in organisms that look very different as adults.
