In the early stages the embryos of four different species will look the same .For example, fish embryos and human embryos both have gill slits. In fish they develop into gills, but in humans they disappear before birth. That what makes them different.
Atoms are the most basic units
Elements are specific types of atoms
Compounds are composed of more than one type of element and are held together by chemical bonds
Prokaryotic cells doesn't have a nucleus.
