The late blastula Xenopus embryo has morphologically distinct regions that are important for understanding gastrulation. These include the marginal zone (shown in red), the veghetal base (big yolky cells shown in yellow) and the animal cap (blue). Note that overlying the marginal zone the superficial cells are colored yellow. That's because in Xenopus the superficial cells of the marginal zone contribute to the archenteron.
(figure adapted from material courtesy of Dr. Ray Keller, Univ. of Virginia)