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coenogenesis
(gcide) | 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. (Zool.) 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] |
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