In vertebrate animals, the embryo is formed by the fusion of the sperm cell from the father and the egg cell from the mother. The resulting embryo is often divided into two different regions, or poles. The upper end is called the animal pole that is made up of small cells that rapidly divide and grow. The lower end of the embryo is called the vegetal pole. It is made up of larger, yolk type cells that divide and grow more slowly. The animal pole is termed that due to the speed at which the cells multiply versus the vegetal pole (animals are faster and more lively than vegetables).
Some scientists believe that the animal pole actually becomes the embryo itself over time. They believe that the vegetal pole eventually becomes protective tissue that provides nourishment for the developing embryo (like a placenta for human embryos).