The generation of ova or eggs, the female gametes. Primordial germ cells, once they have populated the gonads, proliferate and differentiate into either sperm in the testis or ova in the ovary. The decision to produce either motile spermatocytes or more sedentary oocytes is based primarily on the genotype of the embryo. In rare cases, this decision can be reversed by the hormonal environment of the embryo, so that the sexual phenotype may differ from the genotype. Formation of the ovum most often involves substantial increases in cell volume as well as the acquisition of organellar structures that adapt the egg for reception of the sperm nucleus, and support of the early embryo. In histological sections, the structure of the oocyte often appears somewhat random, even chaotic, but as the understanding of its chemical and structural organization increases, an elegant but still somewhat cryptic order begins to emerge.