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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 observations of these stages in different vertebrates, he realised that Johann Friedrich Meckel's recapitulation theory must be wrong. For example, he noticed that the yolk sac is found in birds, but not in frogs. According to the recapitulation theory, such structures should invariably be present in frogs because they were assumed to ...
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 ...
A homonculus inside a sperm cell, as drawn by Nicolaas Hartsoeker in 1695 Jan Swammerdam, Miraculum naturae sive uteri muliebris fabrica, 1729. In the history of biology, preformationism (or preformism) is a formerly popular theory that organisms develop from miniature versions of themselves.
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He set an example for others, from contemporaries Roger Federer or Novak Djokovic or Serena Williams, to members of the following generations, including heir apparent Carlos Alcaraz. “I was a ...
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For example, the brain of a human embryo looked first like that of a fish, then in turn like that of a reptile, bird, and mammal before becoming clearly human. The embryologist Karl Ernst von Baer opposed this, arguing in 1828 that there was no linear sequence as in the great chain of being, based on a single body plan , but a process of ...