According to the recapitulation theory, early human embryos have structures like gill slits, so, therefore, early human embryos represent the form of adult fish, which also have gill slits. Answer. Since Darwin's time, textbooks have reiterated that early embryos of many vertebrates, including humans, have tiny pouches that reflect an evolutionary fish stage. So, the embryo of all chordates does not have a backbone. The development of the human embryo reveals steady progress toward a fully functional human body. The most conspicuous and familiar members of Chordata are vertebrates, but not all have vertebrae. As chordates, all vertebrates have a similar anatomy and morphology with the same qualifying characteristics: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. MEDIUM. Animals in the phylum Chordata share four key features that appear at some stage during their development: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and; a post-anal tail. The so-called gill slits of a human embryo have nothing to do with gills, and the human embryo does not pass through a fish stage or any other evolutionary stage. The vertebrae and nervous systems develop early so all embryos appear to have a tail. The nerve cord, notochord, postnatal tail, and pharyngeal slits are the four characteristics that are present in the chordates. Darwin’s theory of biological evolution noted that all vertebrates have gill slits and tails in early stages of embryo formation, even though these features may be lost or modified in … Explanation: According to embryology, all vertebrates exhibit similar traits and structures at their embryonic stage. It becomes very difficult to differentiate between the embryos of a fish, and that of a bird, or embryo of a fish, and a human. B. Organic evolution. Example: All vertebrates embryos have gill slits, which may become gills or ear bones. C. Metamorphosis. The theory of recapitulation, also called the biogenetic law or embryological parallelism is often expressed in Ernst Haeckel's phrase as "ontogeny recapitulates phylogeny". Embryos of all vertebrates have deep structural similarities and these deep similarities are said to clearly show evidence for evolutionary relationships. All vertebrate embryos follow a common developmental path due to their common ancestry. German zoologist Ernst Haeckel is perhaps most famous for defending evolution with the argument that creatures replay their evolutionary past when developing in the womb. Like all chordates, vertebrates have a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. So what makes a chordate, a chordate? More recently, embryologists … The embryos of birds and mammals clearly show gill-like structures, more technically called pharyngeal arches. look very similar and it is often difficult to tell them apart. Many traits of one type of animal appear in the embryo of another type of animal. All vertebrate embryos follow a common developmental plan due to having a set of genes that gives the same instructions for development. D. Biogenesis. Mayr is not claiming that human embryos actually have the gill slits of a fish. For example, all vertebrate embryos have "gill slits" which are not actually gills. Embryos of many different kinds of animals: mammals, birds, reptiles, fish, etc. Both fish and bird embryos exhibit gill slits and a tail. Reason for incorrect answer: Option a. is given as, “backbone.” Backbone is the property of vertebrates and all chordates are not vertebrates. 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