For example, the notochord is a structure that was present in the first fishes to swim the oceans. It is a thin flexible rod located in these fishes' backs that helped stiffen the body and conserve momentum in the animal's side-to-side swimming motion. Only a few modern species, such as lampreys, have notochords as adults, yet this structure is present in all vertebrate embryos. (In fact, the term "chordate" refers to the notochord, not the spinal cord as may seem to be the case.) Located ventral to the neural tube, it plays an important role in the organization of a developing vertebrate, and elements derived from it appear in the vertebral column.
A principle of biological development is that it proceeds more like a recipe than like a blueprint. Each stage relies on the stage before it. Every species of vertebrate is a variation on the basic vertebrate body plan; thus each vertebrate embryo starts out appearing very much like this basic plan, and the modifications derived through evolution are then applied to it. More closely related species have more of these modifications in common, and these modifications typically appear later in development.
For example, at a particular stage of development, a cat embryo and a human embryo resemble each other more than either resembles a fish embryo, reflecting the fact that cats and humans (both mammals) are more closely related to each other than to fishes. Likewise, at another particular developmental stage, a cat embryo and a dog embryo resemble each other more than either resembles a human embryo, reflecting the fact that cats and dogs (both carnivorans) are more closely related to each other than to humans.