Final answer:
Comparative embryology examines similarities and differences in embryos of different species to understand common ancestry. Vertebrate embryos share features such as gill slits and tails, but most lose them during development. Comparing embryos helps identify patterns and evolutionary relationships.
Step-by-step explanation:
Comparative embryology is the study of the similarities and differences in the embryos of different species. Similarities in embryos are evidence of common ancestry. All vertebrate embryos, for example, have gill slits and tails. Most vertebrates, except for fish, lose their gill slits by adulthood. Some of them also lose their tail. In humans, the tail is reduced to the tail bone. Thus, similarities organisms share as embryos may be gone by adulthood. This is why it is valuable to compare organisms in the embryonic stage.