_Ontogeny_, the history or study of the development of the individual, generally called embryology. In reality the scope of embryology is the ontogenetic study of the various species, and this branch of developmental study alone can be checked by direct, 'exact' observation, for the simple reason that the individuals alone are entities, while the species, genera, families, etc., are abstract ideas. The _ontogenesis of any given living organism is a short, condensed recapitulation of its ancestral history or of its phylogenesis_. This is Haeckel's 'fundamental biogenetic law.' A complete proof of the phylogeny of any creature would be given by the preservation of an unbroken series of all its fossil ancestors. Such a series will in most cases, for obvious reasons, always remain a desideratum. In a few cases, however, the desideratum is nearly met: for example, the ancestral line of the one-toed digitigrade horse from a four-or five-toed plantigrade and still very generalized Ungulate is approaching completion. Phylogenetic study has to rely upon other help. This is afforded by comparative anatomy and by the study of ontogeny. If the latter were a faithful, unbroken recapitulation of all the stages through which the ancestors have passed, the whole matter would be very simple; but we know for certain that in the individual development many stages are left out (or, rather, are hurried through, and are so condensed by short-cuts being taken that we cannot observe them), while other features which have been introduced obscure, and occasionally modify beyond recognition, the original course. Again, the sequence of the appearance of the various organs is frequently upset (_heterochronism_). Some organs are accelerated in their development, while others, 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_, _Coelom_, _Gonochorism_, _Gastrula_, _Metazoa_, _Gnathostomata_, _Acrania_, _Craniota_, and _Amniota_.