For related species, earlier stages of development are usually more similar than later stages. In the early stages of human and chimpanzee development, the fetuses are very similar, but as development proceeds, the developmental pathways diverge.
For a more detailed example, take crustaceans. Crustaceans include critters you've heard of, such as crabs, lobsters, and shrimp. But the group also contains some members that aren't immediately recognizable as being related to crabs and lobsters. Perhaps the best example is the barnacle. Barnacles, believe it or not, are really quite like lobsters. Although an adult lobster (mmm, mmm, good) is quite a bit different from an adult barnacle (not delicious at all), the early larval stages are extremely similar.
In the early stages of development, the different members of a group are extremely similar, but as development proceeds, the specific adult features of each group are expressed. From this fact, researchers discovered that much of the diversification in body structure within the crustacean group results from developmental shifts later in development.
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