Fig. 2.

Patterning of the Xenopus gastrula. (A) The Xenopus gastrula has dorsal and ventral signaling centers, marked here by the expression of Chordin (Chd) and Sizzled (Szl) mRNA, respectively. Each center is regulated by an Activator/Inhibitor pair. The circular structure seen is the blastopore, which eventually becomes the anus. (B) An extracellular network of proteins that regulates the transcription factor Smad1 in the Xenopus gastrula. This self-regulating circular system controls embryonic patterning.¹⁰,¹¹