Fig. 8. The model we propose for control of the second phase Ca²⁺ oscillations by Cdk1 activity in the ascidan. (Top) Sperm-egg fusion and extrusion of first and second polar bodies are shown diagrammatically to indicate relative times of fertilisation, transition from meiosis I to meiosis II and meiotic exit into interphase. The sperm head, coloured in red, depicts the sperm arriving to deliver fully active sperm factor to the egg. (Bottom) A typical pattern of fertilisation-triggered Ca²⁺ oscillations is shown by the black line. Previously determined Cdk1 activity (McDougall and Levasseur, 1998) is indicated by the blue line, and Ins(1,4,5)P₃ responsiveness (Levasseur and McDougall, 2003) by the red line. Hypothetical sperm factor activity is indicated by the pseudocoloured bar, and correlates with Cdk1 activity.