1. Four spermatozoa are produced from each precursor cell in spermatogenesis, but only one ovum is formed from each precursor cell in oogenesis.
2. Spermatogenesis occurs continually and is carried through to completion in about 2 months, meaning that there is always "fresh" sperm. In contrast, the oocytes are all formed before birth and remain in a prolonged prophase of the first meiotic division from fetal life until they are ovulated.