Fig. 269: Principle of Haeckelís recapitulation diagram according to Gould (1977), simplified.
Organisms 1-4 are said to be phylogenetically linked, i.e. direct evolution from 1 to 4 is assumed (e.g. fish - mammal).
The ontogenesis of these four organisms proceeds from A-E, A-G, A-H and B-K. The change in features takes place through the gradual addition of new stages (F-K) to the original ontogenesis (of organism 1).
Individual embryonic or foetal stages are eliminated here (D, later also A); through this others are shifted to an earlier stage of development (stages from E).
In the ontogenesis of organism 4, the appearance of E (e.g. "gill slits ") thus represents the recapitulation of an adult feature of the original organism 1 (e.g. gills).