A fate map of a Xenopus blastula, just before gastrulation begins, shows that the top portion of the embryo will become ectoderm (skin and nerve), the central portion will become mesoderm (bone, muscle, and blood), and the lowest portion will become endoderm (gut). How is it that the endoderm and mesoderm, shown on the outside in the fate map, end up on the inside in the embryo after gastrulation?
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Explanation:
Xenopus laevis: egg
The egg is composed of an animal & a vegetal region, both covered by vitelline membrane.
Meiosis is stopped at 1st division with apparent 1 polar body (the 2nd polar body comes after fertilization).
After fertilization, the cortex (the layer below plasma membrane) rotates to determines future dorsal region at a position opposite to the site of sperm entry.
Xenopus laevis : fertilization and early growth
1. one sperm enters animal region
2. completes meiosis
3. egg and sperm nuclei fuse
4. vitelline membrane lifts
5. yolk rotates to down (15 mins)
6. cortical rotation (60 mins). Cortex is layer below plasma membrane
-rotation determine future dorsal region
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