The growth phase of oogenesis is longer and more complicated than in spermatogenesis. There is considerable enlargement in the size of the primary oocyte during the growth phase. For instance the primary oocyte of frog initially has a diameter of only about 50 microns but at the end of the fruit phase the diameter of the matured egg will be as much as 2000 microns.
Growth in the primary oocyte is brought about by the accumulation of a large amount of fats and proteins. These get accumulated in the form of yolk. The yolk is usually concentrated towards the lower portion of the egg called the vegetal pole. The upper side of the egg which contains cytoplasm and the egg nucleus remains often separate from the yolk and it is called the animal pole.