Fig. 8. Amphistomy, AP axis formation, and Brachyury expression (blue) in the common ancestor of the Bilateria. (A) Following AP axis specification, the vegetal region of the embryo becomes subdivided into an anterior, Brachyury negative, and posterior, Brachyury positive, domains. (B) The blastopore closes in an amphistomous fashion. (C) The posteriorly located growth zone is responsible for the AP axis elongation (arrow). As a result of the whole process, Brachyury is expressed posteriorly and on the ventral midline.