1. Pollination takes place in Pisum sativum between 24 and 36 hours before the open flower stage. Fertilization has occurred and division of the zygote is under way by the time the flower is fully opened. 2. The pollen tube enters the embryo sac between the synergids, neither of which disintegrates until later. 3. One male gamete nucleus fuses with the two polar nuclei. In the division of the primary endosperm nucleus three separate groups of chromosomes are recognizable on the equatorial plate. 4. The primary endosperm nucleus divides before the zygote nucleus. Free nuclear division occurs in the endosperm in the early stages of development. Cell formation later takes place in the micropylar portion of the endosperm, the chalazal portion remaining multinucleate. The endosperm is resorbed in the course of embryo development. 5. During the division of the zygote nucleus, the chromosomes from the two gamete nuclei remain in more or less distinct groups until the equatorial plate stage. 6. The zygote divides...