1.  Gastrulation rearranges the embryo into a triploblastic gastrula. –The embryonic germ layers are the ectoderm, mesoderm, and endoderm. Gastrulation rearranges the blastula to form a three-layered embryo with a primitive gut Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

2. Fate map – vital stain Invagination

3. Involution Epiboly

4. Avian Development. Cleavage is meroblastic, or incomplete. Cell division is restricted to a small cap of cytoplasm at the animal pole. Produces a blastodisc, which becomes arranged into the epiblast and hypoblast that bound the blastocoel, the avian version of a blastula.

5. During gastrulation some cells of the epiblast migrate (arrows) towards the interior of the embryo through the primitive streak Ingression (immigration)

8. Once again, the embryonic membranes – homologous with those of shelled eggs. Chorion: completely surrounds the embryo and other embryonic membranes. Amnion: encloses the embryo in a fluid-filled amniotic cavity. Yolk sac: found below the developing embryo. Develops from the hypoblast. Site of early formation of blood cells which later migrate to the embryo. Allantois: develops as an outpocketing of the embryo’s rudimentary gut. Incorporated into the umbilical cord, where it forms blood vessels.

9. Activation of embryonic genome Mexican axolotl o-mutant strain The “o” gene is a recessive gene “O” gene is the normal, dominant gene In embryos obtained from female axolotls homozygous for gene “o”, development is always arrested during gastrulation. WHY? “O” protein is necessary to activate the embryonic genome. PROOF?

10. The eggs can be rescued by injecting eggs of o/o females with cytoplasm from normal eggs. Where is the "corrective component" (O protein) produced? Rescue Experiments Cytoplasm from Egg vs. Primary Oocytes Cytoplasm vs. Primary Oocyte fluid from germinal vesicle

11. What is the “O” protein doing?

12. activation of the embryonic genome. How can we prove it?

13. When is mammalian embryonic genome activated? Two-cell stage determined by radioactive labeling experiments Imprinting of mammalian gamete’s nucleus Experiments combining: Female pronucleus combined with Female pronucleus Male pronucleus combined with Male pronucleus