ellauri162.html on line 798: Pharyngula is a term used by evolutionists to describe a hypothetical phylotypic stage of development in embryology. It is mistakenly thought by most evolutionists that this stage represents the basic vertebrate body plan in the common ancestor of all vertebrates. There is currently a dispute among scientists as to how similar embryos are and to the reality of this stage.
ellauri162.html on line 800: There are six stages to embryonic development, and the pharyngula stage is towards the middle. In the early stages of development there is significant diversity in the morphology of embryos, this diversity decreases over time till the pharyngula stage where they are most similar (often difficult for anyone but trained embryologist to differentiate), and finally in the last stages of development morphology diversifies again. It is hypothesized that the reason the pharyngula stage is so morphologically constrained is that this is the point where sequential activation of hox genes is initiated so any strong deviations from the developmental plan would lead to drastic changes in the final phenotype of the organism.
ellauri162.html on line 814: The concept of a highly conserved ontogeny dates back to 1828 and the work by Karl von Baer. Baer´s work was cited by Charles Darwin and used in support of his Theory of Evolution. The concept was made famous though by Ernst Haeckel in 1874 with the publication of his drawings of the conserved stage. Haeckel was mainly pushing the concept of recapitulation in which he hypothesized that ontological development repeated the evolutionary steps of the organism. Recapitulation has since been discredited and is not accepted by any modern biologist. Haeckel has been accused of falsifying his embryonic drawings, most notably by Jonathan Wells in his book Icons of Evolution. Some biology text books used Haeckel´s drawings for many years after it was known they were faked. However, most modern biology textbooks only use them now for historical reference and actual photos of embryos are used to discuss the pharyngula stage.
ellauri162.html on line 816: 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.
ellauri162.html on line 818: The Haeckelian form of recapitulation theory is considered defunct. Embryos do undergo a period or phylotypic stage where their morphology is strongly shaped by their phylogenetic position, rather than selective pressures, but that means only that they resemble other embryos at that stage, not ancestral adults as Haeckel had claimed.
ellauri243.html on line 762: 2. The other James Thomson, in full James Alexander Thomson, (born Dec. 20, 1958, Chicago, Ill., U.S.), is an American biologist who was among the first to isolate human embryonic stem cells. Thomson extracted stem cells from human embryos. However, this confronted him with a moral dilemma, as such an extraction is fatal to the embryo. After consulting with several bioethicists at the university, Thomson decided that continued research was ethical as long as the embryos, "created" by couples who "no longer wanted them" in order to "have children", would otherwise be "destroyed anyway." I just love medicinal ethics! Kunnon personismia. Montako neekeriä saa keilata pelastaaxeen yhden valkoisen joka työntää lastenvaunuja.
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