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Essential Cell Biology Item Preview. EMBED for wordpress. Want more? Haeckel formulated his theory as 'Ontogeny recapitulates phylogeny'. The notion later became simply known as the recapitulation theory. Ontogeny is the growth size change and development structure change of an individual organism; phylogeny is the evolutionary history of a species.
Haeckel claimed that the development of advanced species passes through stages represented by adult organisms of more primitive species. For example, Haeckel proposed that the pharyngeal grooves between the pharyngeal arches in the neck of the human embryo not only roughly resembled gill slits of fish, but directly represented an adult 'fishlike' developmental stage, signifying a fishlike ancestor.
Embryonic pharyngeal slits, which form in many animals when the thin branchial plates separating pharyngeal pouches and pharyngeal grooves perforate, open the pharynx to the outside. Pharyngeal arches appear in all tetrapod embryos: in mammals, the first pharyngeal arch develops into the lower jaw Meckel's cartilage , the malleus and the stapes.
Haeckel produced several embryo drawings that often overemphasized similarities between embryos of related species. Modern biology rejects the literal and universal form of Haeckel's theory, such as its possible application to behavioural ontogeny, i. Haeckel's drawings misrepresented observed human embryonic development to such an extent that he attracted the opposition of several members of the scientific community, including the anatomist Wilhelm His, who had developed a rival 'causal-mechanical theory' of human embryonic development.
These differences were, in turn, caused by 'heredity'. His compared the shapes of embryonic structures to those of rubber tubes that could be slit and bent, illustrating these comparisons with accurate drawings. Stephen Jay Gould noted in his book Ontogeny and Phylogeny that His's attack on Haeckel's recapitulation theory was far more fundamental than that of any empirical critic, as it effectively stated that Haeckel's 'biogenetic law' was irrelevant.
Darwin proposed that embryos resembled each other since they shared a common ancestor, which presumably had a similar embryo, but that development did not necessarily recapitulate phylogeny: he saw no reason to suppose that an embryo at any stage resembled an adult of any ancestor. Darwin supposed further that embryos were subject to less intense selection pressure than adults, and had therefore changed less. Modern evolutionary developmental biology evo-devo follows von Baer, rather than Darwin, in pointing to active evolution of embryonic development as a significant means of changing the morphology of adult bodies.
Two of the key principles of evo-devo, namely that changes in the timing heterochrony and positioning heterotopy within the body of aspects of embryonic development would change the shape of a descendant's body compared to an ancestor's, were however first formulated by Haeckel in the s. These elements of his thinking about development have thus survived, whereas his theory of recapitulation has not.
The Haeckelian form of recapitulation theory is considered defunct. Embryos do reflect the course of evolution, but that course is far more intricate and quirky than Haeckel claimed. Different parts of the same embryo can even evolve in different directions.
As a result, the Biogenetic Law was abandoned, and its fall freed scientists to appreciate the full range of embryonic changes that evolution can produce—an appreciation that has yielded spectacular results in recent years as scientists have discovered some of the specific genes that control development.
Curiosity, if nothing else, should drive us to study cell biology; we need to understand cell biology to understand ourselves. We are made of cells, we feed on cells, and our world is made habitable by cells.
The challenge for scientists is to deepen our knowledge of cells and find new ways to apply it. All of us, as citizens, need to know something of the subject to grapple with the modern world, from our own health affairs to the great public issues of environmental change, biomedical technologies, agriculture, and epidemic disease.
Cell biology is a big subject, and it has links with almost every other branch of science. The study of cell biology therefore provides a great scientific education.
However, as the science advances, it becomes increasingly easy to become lost in detail, distracted by an overload of information and technical terminology. In this book we therefore focus on providing a digestible, straightforward, and engaging account of only the essential principles.
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