A subset of cells undergo these changes. The animal hemisphere of a Xenopus oocyte is less dense than the vegetal hemisphere. The animal hemisphere has the nucleus as well as some pigments, whereas the vegetal hemisphere has yolk proteins.
Fertilization causes cortical rotation and thereby creates bilateral symmetry.
Animal hemisphere cells give rise to ectoderm, and vegetal cells give rise to endoderm. Mesoderm comes from cells between the future endodermal and ectodermal cells. The brain comes from anterior dorsal ectodermal tissue. Raymond is trying to map the developmental fates of different cells in an amphibian species. To do this, he cuts a 16-cell embryo into clumps of tissue, cultivates the clumps of cells, and observes how they develop.