coenogenesis

Recapitulation Re`ca*pit`u*la"tion (r[=e]`k[.a]*p[i^]t"[-u]*l[=a]"sh[u^]n), n. [LL. recapitulatio: cf. F. recapitulation.] 1. The act of recapitulating; a summary, or concise statement or enumeration, of the principal points, facts, or statements, in a preceding discourse, argument, or essay. [1913 Webster]

2. (Zo["o]l.) That process of development of the individual organism from the embryonic stage onward, which displays a parallel between the development of an individual animal (ontogeny) and the historical evolution of the species (phylogeny). Some authors recognize two types of recapitulation, {palingenesis}, in which the truly ancestral characters conserved by heredity are reproduced during development; and {cenogenesis} ({kenogenesis} or {coenogenesis}), the mode of individual development in which alterations in the development process have changed the original process of recapitulation and obscured the evolutionary pathway. [PJC]

This parallel is explained by the theory of evolution, according to which, in the words of Sidgwick, "the 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." Examples of recapitulation may be found in the embryological development 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 succession 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 region shows gill-slits, like those which in a shark are a permanent feature, and the heart is two-chambered or fish-like. Further development 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 characteristic 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 "rudimentary 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 complete, however, for not all the stages are reproduced in every case, so far as can be perceived; and it is irregular and complicated in various ways among others by the inheritance of acquired characters. The most special students of it, as Haeckel, Fritz M["u]tter, Hyatt, Balfour, etc., distinguish two sorts of recapitulation {palingenesis}, exemplified in amphibian larvae and {coenogenesis}, the last manifested most completely in the metamorphoses of insects. Palingenesis is recapitulation without any fundamental changes due to the later modification of the primitive method of development, while in coenogenesis, the mode of development has suffered alterations which obscure the original process of recapitulation, or support it entirely. --Encyclopedia Americana, 1961. [PJC]