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Human embryos don't have gill slits
- While it may indeed be misleading to refer to them as gills proper, few scientists or textbooks actually make that particular mistake, and the structures in question are indeed colloquially referred to as gill slits. Gill slits are simply the invaginations between the pharyngeal pouches. The pouches contain mesodermal tissue, the gill arches or gill bars. The term is legitimate because the slits do establish contact between the outside and the pharynx. The fact that the arches later develop into non-gill structures is irrelevant.
- The gill arches themselves were not actually discovered by Haeckel, but some three decades before Darwin by a Baltic-German embryologist Heinrich Rathke in his two-volume treatise on human and animal development: Abhandlungen zur Bildungs- und Entwicklungs-Geschichte der Menschen und der Thiere. The existence of these structures is not in dispute and they clearly demonstrate an important homology in vertebrate development.
- They could reasonably be referred to as proto-gills, since they aren't actually used for respiration (even in fish, at that age), and yes, human embryos have them. Also, proto-tails. The point of the original claim, from an "evolutionist" perspective, is that all chordate embryos look essentially identical, and don't begin to resemble their parent species until later in development. This is true, and is an unambiguous indication that all chordates are of common descent.
- Objections to calling them â€œgill slitsâ€ are raised only to prevent the learning of the idea that these show common descent due to their being the structures that develop into gills in adult fish and some amphibian species. Itâ€™s well known, and well established based on evidence that these same gill slits develop into gills in fish and are reabsorbed in mammals. A similar but much more evolutionarily recent example is the growth of embryonic teeth in baleen whales and in hippos. Here, calling them â€œteethâ€, even though they are never used for chewing in whales, is fine â€“ thatâ€™s what they are used for in species where they develop into adulthood. Just to make it a bit more accurate, maybe call them â€œgill pouchesâ€ â€“ since there does not appear to be a perforation. Either way though, the important thing is to preserve the important and well supported fact that these do show common descent, because the same things develop into gills in species where they develop into adulthood.
- "The pharyngeal arches and clefts are frequently referred to as branchial arches and branchial clefts in anology with the lower vertebrates, [but] since the human embryo never (emphasis added) has gills called 'branchia', the term pharyngeal arches and clefts has been adopted for this book." Langman, Jan. Medical Embryology (Baltimore: Williams & Wilkins, 1975), p. 262.
- As noted in the Introduction to the Centennial Edition of Darwin's Origin of the Species: "When the 'convergence' of embryos was not entirely satisfactory, Haeckel altered the illustrations of them to fit his theory. The 'biogenetic law' as proof of evolution is valueless." Thompson, W.R., "Introduction" to Everyman's Library (#811) Darwin's The Origin of the Species (New York: E.P. Dutton & Co., 1956), p. xvi. There are other, far more compelling examples of evolutionary proofs, such as genome comparisons.
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- Agassiz, Louis, 1874. Evolution and Permanence of Type, reprinted in Hull, David L., 1973, Darwin and His Critics, p. 440.
- Talk origins archive
- Gilbert, Scott F., 1988. Developmental Biology, 2nd ed., Sinauer Associates, Inc. Sunderland MASS.