Hence, Darwin saw that one could infer an evolutionary change from a developmental one. For instance, the larval zebrafish has a notochord, an unsegmented rod-like structure, where the adult zebrafish has a segmented vertebral column. Von Baer's rule is obeyed. We can use Von Baer's rule to help construct phylogenies, to make cladograms. This is because by the rule we can make a hypothesis about polarity - which character state is primitive and which is derived.
Notochords are present in the embryo of all vertebrates.
Hence a notochord is a primitive character. A vertebral column is a derived one. Von Baer also provides us with an important connection between development and evolution and we want to understand more about just why there is such a rule. Further, we saw the PJH view of the vertebrate Bauplan in PJH Fig. 2-7 . We will come back later in the course to the issue of what MA might have looked like.