The progressive events taking place in the nucleus from the intact germinal vesicle stage to the second meiotic metaphase will be referred to here as oocyte maturation. This maturation, which is under the influence of gonadotrophins, is a prerequisite for rendering an oocyte fertilizable by a spermatozoon. Whether the subsequent cleavage and development is normal relies heavily on the existence of cortical granules and on the secretion of their contents at the moment of sperm attachment. The development of a defense mechanism to avoid polyspermy is initiated during the oocyte growth phase. Even though in most cases the spermatozoon is incorporated entirely during its penetration process, the question remains if in addition to the nucleus, other organelles entering with it remain physiologically active. In this regard, the mitochondria with their known mechanism for self-replication, and centrioles, which in somatic cells duplicate themselves, are of particular interest. The developmental changes of some organelles will be discussed under normal and certain pathologic conditions. The more recent findings of virus-like “A” particles will also be presented.