Figure 1.

Organizer formation and fate map of Xenopus embryos (Upper panel). In an unfertilized egg, the maternal determinants (small vesicles) are located in the vegetal hemisphere. The sperm entry triggers the fertilization process and small vesicles associated with kinesin (motor protein) moved toward the opposite site (the plus end of microtubules) of the sperm entry point. These vesicles contain the components of Wnt signaling, which establishes the Nieuwkoop center and Spemann organizer during the cleavage stages of early embryogenesis. The lower panel shows the Hensen’s node (Chick embryo), embryonic shield (Fish embryo), and node/AVE (Mouse embryo).