FERTILIZATION:

1. Oocyte (actually 1.2 mm diameter) developes into a fertilizable egg after addition of progesterone (progesterone structure shown-see four connected rings)
2. Note mature, fertilizable egg has a white spot
3. Add sperm, which binds to a receptor ("M") in egg
4. Phospholipase C (PLC) is activated and PIP2 is cleaved into DAG and IP3
5. IP3 releases intracellular calcium (from ER or endoplasmic reticulum); increased calcium is responsible for fertilization and later developmental events
6. 90 min after fertilization, zygote cleaves to two cells then cleaves again 30 min later (to 4 cells)