Having learned that they are unable to conceive a child, Monica (Courteney Cox) and Chandler (Matthew Perry) seek out the services of a sperm bank. This is dispiriting enough, but the situation becomes downright disgusting for Chandler when he meets potential donor Zack (played by guest star John Stamos). Meanwhile, Phoebe (Lisa Kudrow) prepares to go out with her ex-boyfriend Mike, when who should show up but another of her ex-boyfriends, the geeky-but-lovable scientist David (Hank Azaria). And elsewhere, an upcoming speech at a paleontology convention serves as the catalyst for a confrontation between Rachel (Jennifer Aniston) and Charlie (Aisha Tyler). ~ Rovi