Gene expression patterns can be used as characters to assign evolutionary homology. The understandings about Bauplan on which we have focusing, beginning last week, have come from comparing gene expression in embryos. Important new ideas include why birds have more vertebrae in their necks than do mammals, why snakes have no body region corresponding to a neck, and possibly no forelimb, what part of the body of the vertebrate sister group corresponds to the vertebrate head, and what amphioxus cells correspond to vertebrate neural crest cells. These are important ideas. We continue today with the same theme, pushing understanding of vertebrate body patterning further down the evolutionary tree.
Some vertebrates appear to depend on pristine habitats such as ancient forests.
We now can consider the archetypal Bauplan as MA s body plan, a craniate, bilaterally symmetrical segmented animal. Ideas about the details of invertebrate phylogenies are not generally agreed to, but the general pattern shown in PJH Fig. 2-2 is a reasonable current idealization. This follows von Baer s rule. A line through the same plane at right angles to the AP axis is the Dorsoventral axis.