| Abstract: | The chronology and dynamics of the female germ cell development, of the mitotic activity of oogonia, and of the chromosome rearrangements at prophase I of meiosis have been quantitatively estimated in 30 cow embryos and foetuses at the age of 1.5 to 9 months. The sexual differentiation of the gonads was shown in a 1.5 month old embryo. The oocytes at the stages of preleptotene chromosome condensation and decondensation occurred in the 1.5 month old embryos and their maximum number was observed in the 2-5 month old foetuses. The leptotene oocytes were found in the 2-2.5 month old foetuses. The transition to zygotene and pachytene was also recorded in the 2-2.5 month old foetuses but their maximum number was observed in the 4-6 month old foetuses; their number was reduced to single oocytes thereafter. The first diplotene oocytes appeared in the 3 month old foetuses but the active transition of the oocytes to diplotene was observed after four months of development. The formation of a layer of follicle cells takes place around the diplotene oocytes. The vast majority of degenerating germ cells are the oocytes in zygotene-pachytene and in diplotene. The population of germ cells is formed by the mitotic division of oogonia in the cow foetuses, mainly at the age of 1.5 to 4 months of development. |
