which we know to be phylogenetically older, are retarded in making their reappearance in the embryo.
These disturbing or distorting newly introduced features or factors show themselves chiefly in connection with the embryonic conditions of growth—for example, yolk-sac, placenta, amnion. They all come within the category of cænogenesis: they are cænogenetic, while the true, undisturbed recapitulation is palingenetic.
Lastly, some features, so-called rudimentary or vestigial organs, instead of disappearing, are most tenacious in their recurrence, while others of originally fundamental importance scarcely leave recognisable traces, and are, so to speak, only hinted at during the embryonic growth of the creature we happen to study. Hence arises the philosophical study of 'Dysteleology.'
Among other terms invented by Haeckel, and now in general use, are Metamere, Metamerism, Cœlom, Gonochorism, Gastrula,
