The ovaries of fetal and neonatal mules and hinnies were examined in order to study the development of the germ cells. Although migration of these cells into the fetal gonad and their subsequent mitotic divisions were apparently normal, most oogonia died in early neonatal life as they entered meiosis. This was thought to be due to the inability of the paternal and maternal sets of chromosomes to form homologous pairs. However, a few germ cells were able to enter meiosis and become oocytes, and possible mechanisms for this are discussed. These few surviving oocytes probably give rise to the occasional Graafian follicles found in the ovaries of adult mules, but it seems highly unlikely that the ova would ever be capable of normal fertilization and development.
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