RECAPITULATION, or BIOGENETIC LAW, in zoology, terms applied to the parallel which exists between the embryonic develop ment of an individual animal (ontogeny) and the historical evolution of its race (phylogeny). This parallel is explained by the theory of evo lution (q.v.), according to which, in the words of Sidgwick, developmental history of the individual appears to be a short and simplified repetition, or in a certain sense a recapitulation, of the course of development of the species' This class of facts was recognized more or less completely by the earliest naturalists, and was extensively studied and formulated by Von Baer and Agassiz previous to the explana tion of them by means of the doctrine of descent; but it should be added that obscurities still exist, concerning the explanation of which embryologists are in doubt or at variance. Ex amples may be found in the embryological de velopment of all vertebrates. Thus the frog develops through stages in which the embryo just before hatching is very fish-like, after hatching becomes a tadpole which exhibits many newt-like characters; and finally reaches the permanent frog stage. This accords with the comparative rank of the fish, newt and frog groups in classification; and also with the suc cession appearance of these groups. Man, as the highest animal, exhibits most completely these phenomena. In the earliest stages the human embryo is indistinguishable from that of any other creature. A little later the cephalic legion shows gill-slits, like those which in a shark are a permanent feature, and the heart is two-chambered or fish-like. Further develop ment closes the gill-slits, and the heart changes to the reptilian type. Here the reptiles stop,
while birds and mammals advance further; but the human embryo in its progress to the higher type recapitulateS and leaves features character istic of lower mammalian forms — for instance, a distinct and comparatively long tail. exists. Most of these changes are completed before the embryo is six weeks old, but some traces of primitive and obsolete structures persist throughout life as ((vestiges* or organs,* and others appear after birth in infancy, as the well-known tendency of babies to turn their feet sideways and inward, and to use their toes and feet as grasping organs, after the manner of monkeys. This recapitulation of ancestral characters in ontogeny is not com plete, however, for not all the stages are repro duced in every case, so far as can be perceived; and it is irregular and complicated in various ways among others by the inheritance of ac quired characters. The most special students of it, as Haeckel, Fritz Muller, Hyatt, Balfour, etc., distinguish two sorts of recapitulation ((paha genesis?" exemplified in amphibian larvae and lccenogenesis,* the last manifested most com pletely in the metamorphoses, of insects. Palin genesis is recapitulation without any funda mental changes due to the later modification of the primitive method of development, while in ceenogenesis, the mode of development has suf fered alterations which obscure the original process of recapitulation, or support it entirely. See EMBRYOLOGY, and consult authorities men tioned thereunder.