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When the human embryo, for instance, is but a simple vesicle, it is an infusorian; when it has gained a liver, it is a mussel; with the appearance of the osseous system, it enters the class of fishes; and so forth, until it becomes a mammal and then a human being.
Human embryology is the study of this development during the first eight weeks after fertilization. The normal period of gestation (pregnancy) is about nine months or 40 weeks. The germinal stage refers to the time from fertilization through the development of the early embryo until implantation is completed in the uterus .
From his studies of comparative embryology, Baer had believed in the transmutation of species but rejected later in his career the theory of natural selection proposed by Charles Darwin. He produced an early tree-like branching diagram illustrating the sequential origins of derived character states in vertebrate embryos during ontogeny that ...
The theory of recapitulation, also called the biogenetic law or embryological parallelism—often expressed using Ernst Haeckel's phrase "ontogeny recapitulates phylogeny"—is a historical hypothesis that the development of the embryo of an animal, from fertilization to gestation or hatching (), goes through stages resembling or representing successive adult stages in the evolution of the ...
Romanes' 1892 copy of Ernst Haeckel's allegedly fraudulent embryo drawings (This version of the figure is often attributed incorrectly to Haeckel.) [1]. Haeckel's illustrations show vertebrate embryos at different stages of development, which exhibit embryonic resemblance as support for evolution, recapitulation as evidence of the Biogenetic Law, and phenotypic divergence as evidence of von ...
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Lacuna (music), an intentional, extended passage in a musical work during which no notes are played; Scientific lacuna, an area of science that has not been studied but has potential to be studied; Lacuna or accidental gap, in linguistics, a word that does not exist but which would be permitted by the rules of a language