ANATOMY

Detailed explanation-1: -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 the adult-form phenotype.

Detailed explanation-2: -In the embryos of all vertebrates, the presence of gill slits supports the theory of recapitulation (repeating the early stages of embryogenesis in earlier evolved animals.)

Detailed explanation-3: -The early embryos of animals as different as chickens and fish had similar vessel branching patterns and slits in their necks, for example. As fish developed, these vessels aligned with gill slits. As chickens developed, the slits closed up and the vessels rearranged and became associated with lungs.

Detailed explanation-4: -Scientists compare the structures of a chicken embryo and a human embryo at the same stage of development. They find that both vertebrates have similar shapes and structures, including the presence of gill slits and tails.

Detailed explanation-5: -Many traits of one type of animal appear in the embryo of another type of animal. For example, fish embryos and human embryos both have gill slits. In fish they develop into gills, but in humans they disappear before birth.