This study by Gregory Pincus and E. V. Enzmann investigates the activation and fertilization of ovarian eggs in rabbits. The research is divided into two parts: the mechanism controlling the first meiotic division and the capacity for fertilization of ovarian eggs. Key findings include: 1. **Mechanism of the First Meiotic Division**: The study shows that the first polar body is formed within 8 hours after mating, with the nuclear membrane dissolving and tetrads forming. This process is similar when ova are cultured in vitro with pituitary hormones or thyroxin. 2. **Fertilization Capacity**: Ovarian eggs can be successfully fertilized even if they have not undergone the first maturation division. In vitro insemination of ovarian eggs results in sperm penetration and, in some cases, the formation of a male pronucleus. Transplantation of fertilized ovarian eggs into pseudopregnant rabbits leads to regular cleavage. 3. **Role of Follicular Cells**: The study suggests that follicular cells may either maintain the egg in a state where nuclear maturation is impossible or supply substances that inhibit it. The isolation of ova in vitro results in a similar sequence of nuclear changes, indicating that follicular cells play a crucial role in maintaining the egg's nutritional state and inhibiting nuclear maturation. 4. **Implications**: The findings suggest that mammalian ovaries contain many fertilizable ova that never emerge from the ovary. This opens up new possibilities for studying mammalian eggs in vitro, overcoming the limitation of limited egg availability. The study concludes that ovarian eggs in ripe follicles of mammals are activatable and fertilizable at any time, providing insights into the fundamental nature of the activation process.This study by Gregory Pincus and E. V. Enzmann investigates the activation and fertilization of ovarian eggs in rabbits. The research is divided into two parts: the mechanism controlling the first meiotic division and the capacity for fertilization of ovarian eggs. Key findings include: 1. **Mechanism of the First Meiotic Division**: The study shows that the first polar body is formed within 8 hours after mating, with the nuclear membrane dissolving and tetrads forming. This process is similar when ova are cultured in vitro with pituitary hormones or thyroxin. 2. **Fertilization Capacity**: Ovarian eggs can be successfully fertilized even if they have not undergone the first maturation division. In vitro insemination of ovarian eggs results in sperm penetration and, in some cases, the formation of a male pronucleus. Transplantation of fertilized ovarian eggs into pseudopregnant rabbits leads to regular cleavage. 3. **Role of Follicular Cells**: The study suggests that follicular cells may either maintain the egg in a state where nuclear maturation is impossible or supply substances that inhibit it. The isolation of ova in vitro results in a similar sequence of nuclear changes, indicating that follicular cells play a crucial role in maintaining the egg's nutritional state and inhibiting nuclear maturation. 4. **Implications**: The findings suggest that mammalian ovaries contain many fertilizable ova that never emerge from the ovary. This opens up new possibilities for studying mammalian eggs in vitro, overcoming the limitation of limited egg availability. The study concludes that ovarian eggs in ripe follicles of mammals are activatable and fertilizable at any time, providing insights into the fundamental nature of the activation process.