Proper development and function of the ovary is vital for female mammalian reproduction. The development of the ovary plays an important role in the formation of the ovarian follicle reserve, which governs the female reproductive lifespan. One of the main functions in the developed ovary is the maturation of ovarian follicles, a process known as folliculogenesis. Exposure to reproductive toxicants and reproductive diseases can cause adverse reproductive outcomes, possibly leading to infertility. The purpose of this thesis was to construct a model of early ovarian development, spanning the origin of primordial germ cells to the formation of primordial follicles, in mice and to gather data to be used for the development of a model of the preantral stages of folliculogenesis, the generation of secondary follicles from primordial follicles, in rhesus macaque.