Some web sites to look at   From Scott Gilbert's excellent textbook: section about the gray crescent (which has been mentioned in class but not fully discussed). http://worms.zoology.wisc.edu/frogs/ This is an excellent site from the University of Wisconsin on amphibian embryology. Unfortunately many of the videos don't work on some computers, including our more recent Macs running Safari, Chrome or Firefox. But please try them yourselves, and let us know whether they work for you (and what browser and operating system you're using). If we get enough data, we will contact the managers of the site. Please send your comments to Elizabeth Harris, chlamy@duke.edu In particular, please look at these links: the gray crescent salamander gastrulation another on gastrulation fate mapping by Vogt (drawings) Frogs in Space (NASA and Xenopus; weightlessness that NASA calls "Microgravity") The Differential Adhesion "Thermodynamic" Hypothesis, very persuasively explained by Scott Gilbert's textbook.