The endoplasm is the fluid portion of the cytoplasm and has the same characteristics as the cytoplasm of a somatic cell. On the other hand, the cortex is a thicker, almost rigid gel that contains cortical granules and pigment granules that are not found in the endoplasm. Cortical granules are round, membrane bound organelles that contain proteins and acid mucopolysaccharides. These structures do not perform any function until fertilization when they are released into the area surrounding the egg cell itself. Pigment granules in many cases contain melanin and provide the coloring for the light and dark regions of the oocyte.
Mitochondria exist in very few numbers during the early stages of oogenesis.
Nevertheless, at the end of oogenesis, when the egg is completely formed, the number of mitochondria is extremely high. Some may contain as many as 100,000 times more mitochondria than a somatic cell. This large number exists so that once the egg is fertilized the embryo can use the preexisting mitochondria, until it is able to produce its own. Also, because of the incredible amount of mitochondria in the cell, there is a dramatic increase in the amount of mitochondrial DNA in the ooplasm. The cristae of oocyte mitochondria are also very similar to those of somatic cells, except in a few highly modified cells that contain a different organization of cristae.