p631fig17.05 How a blastula develops into a gastrula.  See the text for details.<BR>||  1. The vegetal pole of the blastula flattens, [Animal, vegetal] hemisphere, blastocoel.  2. Some cells change shape and move inward to form the archenteron, Elastopore.  3. Other cells break free, becoming mesenchyme. 4. Then extensions of mesenchyme cells attach to the overlying ctoderm, Archenteron.  5. The archenteron elongates, assisted by the contraction of mesenchyme cells.  6. The mouth will form, where the archenteron meets ectoderm. 7. The blastopone will form the anus of the mature animal. [Mesenchyme, Ectoderm, Endoderm, Blastocoel, Archenteron, Mesenchyme]. Concept 38.3, www.macmillanhighered.com