Question

the study of embryos and other early stages of development can help in the classification of living things, past and present, by providing clues about the - prokaryotic species that are at risk of extinction. barriers to mating among marine populations. rate of appearances of terrestrial organisms. common ancestry among vertebrate species.

Explanation:

Embryology studies early - stage development. By comparing embryos of different vertebrate species, similarities can be found which suggest common ancestry. Prokaryotic species (non - vertebrate, simple cells), mating barriers (reproductive isolation aspect not directly from embryo study), and rate of appearance (more about fossil record/time - scale in some cases) are not the main focus here. Embryo study helps in showing common ancestry among vertebrates.

Answer:

common ancestry among vertebrate species.