Germ cell proliferations in the fetal horse ovary.

During the 340 day pregnancy of the horse, the germ cells in the fetal ovary showed a meiotic prophase which began in days 60-70 and might be prolonged after day 200. Three or four successive oogonial mitotic proliferations passed into the meiotic prophase but the great majority of the oocytes first involved degenerated, and no appreciable numbers of primordial follicles were left behind. At 150 days of pregnancy and again at 197 days, oocytes in early meiotic stages filled the ovarian cortex. Primordial follicles were present, but rare. As the prophase gradually came to an end, groups of oocytes became enclosed by small 'boundary' cells which formed in the stroma. Coalescence of these groups led to the appearance of coiled and branching formations containing small and large oocytes. Many of the original groups, however, contracted and became narrow strands and these persisted until the end of pregnancy, many oocytes disappearing from them, while others eventually developed in primordial follicles.