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 can infer an evolutionary change from a developmental one. For example, limb buds develop structures of the limb in a proximodistal sequence.
Shoulders develop first, digits develop last.
In the example cladogram digits appear, as specializations, derived features in Acanthostega . We can use it to assign polarity. Evolution goes from no digits to possessing digits, not the other way round. Von Baer provides us with an important connection between development and evolution and we want to understand more about just why there is such a rule. This is the next subject we take up.