Therefore, the early embryo of a higher animal is never like a lower animal, but only like its early embryo. The first 2 of the generalizations, together, are important to us as Von Baer's Rule. The change can be observed directly by studying the embryo. Hence, Darwin saw that one could infer an evolutionary change from a developmental one. For example, as we shall examine in detail later, limb buds develop structures of the limb in a proximodistal sequence.
Shoulders develop first, digits develop last.
Von Baer's rule is followed. We can use Von Baer's rule to help construct phylogenies, to make cladograms. Recall that polarity, deciding which character state is primitive and which is derived, can be established by outgroup analysis. We can use Von Baer's rule in the same way. If the rule is followed, the character-state that is primitive appears earlier in development.