Most of the gene products are for sperm motility and binding to the egg. In the spermatid, the sperm nucleus becomes condensed, and histones are replaced with protamines. Capacitation is not yet fully understood chemically, but it is needed for the acrosome reaction to occur at fertilization. Capacitation can be mimicked by incubating sperm from the testis in tissue culture medium or in fluids from the oviducts. Nevertheless, most individuals arise from fertilized eggs.
Sex greatly increases variability and is usually selected for.
In the human being, o gonia have formed from primordial germ cells by the second month and increase in numbers by mitosis between the second and fifth months after conception. Some o gonia enter meiosis as early as 12 weeks and arrest as primary o cytes in first meiotic prophase. By birth, all remaining germ cells are arrested primary o cytes which number about 700,000 to 2 million. Most of these become atretic during childhood, so at puberty only about 20,000 to 40,000 oocytes remain in the ovaries. They sit in arrested first meiotic prophase for 12 to 40 years.