Qualitative observations of forty-nine bovine foetuses taken at varying stages of gestation revealed the following: (1) the germinal ridge is established at the 32nd day of gestation; (2) due to the formation of the tunica albuginea of the primitive testis, foetal sex is distinguishable by Day 39; (3) meiosis begins in ovaries at 75 to 80 days post coitum; (4) oogonial mitosis is discontinued by 150 to 170 days; (5) the majority of the oocytes have attained their stage of rest (pachytene) and are enveloped in primordial follicles by Day 170; and (6) vesicular follicles appear in ovaries at Day 250 of gestation.Total germ-cell numbers, based on volumetric determinations or direct counts, increased from a low of approximately 16,000 at Day 50 to a high of 2,700,000 at Day 110. From Day 110 numbers declined abruptly to 107,000 at Day 170 and the ovaries of four foetuses at 270 days of gestation (average length of gestation, 283 days) contained an average of 68,000 \m=+-\ 21,000 germ cells.Seventy prenatally irradiated (400 R, whole-body to dam) and twenty-five non-irradiated heifers were slaughtered at either 9 or 23 months of age. Microscopic examinations of their ovaries revealed that irradiation was apparently without effect during the interval of 17 to 59 days of gestation, and of questionable effect during the interval of 59 to 119 days of gestation. Irradiation administered during the 119-to 154-day interval, however, diminished germ-cell numbers by an average of 68%. From Day 154 to term, irradiation was again without effect. The mitotically active oogonium was probably the cell affected during the interval of 59 to 119 days of gestation and the drastic germinal decline during the 119-to 154-day interval was believed to be related to the progression of oocytes from zygotene to pachytene, a transition during which the oocyte under normal conditions is highly susceptible to death.

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Gametogenesis and ovarian development were examined by means of histologic, cytologic and radioisotopic techniques in porcine ovaries of ages ranging from 13 days post coitum (PC) to birth (114 days PC). Limited observations were made i n postnatal ovaries (birth to 130 days post parturn).Primordial germ cells were noted at 18 days PC in the region of the germinal ridge (border of mesonephros) seven days prior to its formation (24-25 days). Meiosis began as early as day 40 in some cases and was a characteristic of most ovaries by day 50.Diplotene, the resting stage of porcine oocytes, first appeared at 50 days PC and 99% of all germ cells were at diplotene by 20 days p o s t partum. Both the paucity of oogonia and absence of oogonial mitoses at day 100 PC signaled the end of oogenesis.

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