<P> Oogenesis starts with the process of developing oogonia, which occurs via the transformation of primordial follicles into primary oocytes, a process called oocytogenesis . Oocytogenesis is complete either before or shortly after birth . </P> <P> It is commonly believed that, when oocytogenesis is complete, no additional primary oocytes are created, in contrast to the male process of spermatogenesis, where gametocytes are continuously created . In other words, primary oocytes reach their maximum development at ~ 20 weeks of gestational age, when approximately seven million primary oocytes have been created; however, at birth, this number has already been reduced to approximately 1 - 2 million . </P> <P> Recently, however, two publications have challenged the belief that a finite number of oocytes are set around the time of birth . The renewal of ovarian follicles from germline stem cells (originating from bone marrow and peripheral blood) has been reported in the postnatal mouse ovary . In contrast, DNA clock measurements do not indicate ongoing oogenesis during human females' lifetimes . Thus, further experiments are required to determine the true dynamics of small follicle formation . </P> <P> The succeeding phase of ootidogenesis occurs when the primary oocyte develops into an ootid . This is achieved by the process of meiosis . In fact, a primary oocyte is, by its biological definition, a cell whose primary function is to divide by the process of meiosis . </P>

When does the first meiotic division in oogenesis occur