How is the social cast created in The Brave New World?

When a person is still in his embryonic state, he is in a bottle with thousands of other bottled embryos. The workers at the DHC put alcohol into his bottle, and the more alcohol they put in, the lower his class will be, and the shorter and dumber he himself will be. The embryos are all predestined in this way. Then, when they are children, they listen to hypnopaedic messages that tell them to love the cast they've been put into.