Vitellogenesis is the stage where much of the oocyte growth occurs. The completion of second maturation division (meiosis), which may occur after ovulation, results in ripe egg or ovum. In some species such as sea urchins and frogs, the female produces hundreds or thousands of eggs at a time where as in most mammals including humans only a few eggs are produced in the entire lifetime of the individual. In this section we shall discuss oogenesis in two groups of animals,
(1) The amphibians as an example of organisms which produce yolky eggs and
(2) Mammals, as an example of organisms whose eggs do not contain any yolk.