The occurrence of eggs needs to go through the following three stages.

① The proliferation of oogonia and the formation of primary oocytes. After embryonic differentiation, the sex primordial cells and oogonia on the ovaries of female fetal calves undergo mitosis to produce a large number of primary oocytes

② The first maturation division of primary oocytes and the formation of secondary oocytes. When the primary oocyte is formed, the first maturation (reduction) meiosis begins immediately, and is then surrounded by follicular cells in the ovaries, so that the maturation division of the primary oocyte stops at the early diplotene stage, and the first dormancy occurs. Until the arrival of the cow's first estrus, that is, before the first ovulation, under the action of the LH peak in the blood, the primary oocyte is released from dormancy and resumes maturation division, which is called re-starting. The first maturation division produces a polar body (first polar body) and a secondary oocyte, followed by ovulation;

③ The second maturation division and the formation of haploid eggs. The secondary oocyte enters the fertilization site of the fallopian tube and begins the second mature division, and then dormant in the middle stage, waiting for the sperm to enter the egg. The cattle are observed by B-ultrasound until the oocyte enters the zona pellucida and activates the oocyte, and then finally completes the second mature division, produces the second polar body and haploid egg, and fertilizes with sperm.