Karl Ernst von Baer (1792 - 1876) defined his first embryological law as follows:
"The general features of a large group of animals appear earlier in the embryo than the special features."
Cartilage, for example, is found in all fish - in cartilaginous fish such as sharks as well as in bony fish like these salmon pictured opposite. Cartilage is a general character; bone is a special character, being found only in bony fish. Von Baer's law correctly predicts that, in bony fish, cartilage will appear earlier in individual development, and will transform into bone.
The characters von Baer called 'general' are in evolutionary terms ancestral; and his 'special' characters are evolutionarily derived: the successive transformations from general to special forms of a character are evolutionary changes between ancestral and derived character states. This can be useful for phylogenetic inferences.
While von Baer's law is widely accepted to have some truth, it is also known to have exceptions. It is closely related to the theory of recapitulation.