SUMMARYThe early stages in the development of the oögonium and the antheridium ofSclerospora graminicolawere found to be similar to those described for other members of the Peronosporales. A distortion, which was not attributed to poor fixation, was characteristic of nuclei entering the young oögonium. There were at least two divisions of most, if not all, of the nuclei in the oögonium and antheridium. The fusion of the two sexual nuclei took place very slowly, so that they had not yet fused when the oöspore had reached the resting stage. The haploid number of chromosomes was found to be present at metaphase of all mitoses in the developing sex organs.After the nuclei in the oögonium ceased to divide, all except one were found in the periplasm. The one functional female pronucleus remained within the dark mass of cytoplasm in the center of the oôplasm. The conjugation tube penetrated into the oöplasm and one nucleus left the antheridium, passed through the conjugation tube and into the oöplasm. After the entrance of the male pronucleus, the wall of the oöspore was formed from the oöplasm and at the same time a large central body was formed. In the mature oöspore the wall of the oögonium became wrinkled and came into contact with the wall of the oöspore. The two walls were not observed to have become continuous at any point.A few hours after the oöspores had been placed in the conditions under which they germinated the fusion nucleus divided. Subsequent divisions followed until the oöspore contained typically about 32 nuclei. At late prophase of the division of the fusion nucleus the chromosomes were seen to be associated in pairs. At anaphase the haploid number of chromosomes was found to go to each pole. Late prophase stages of dividing nuclei found during the later development of the germinating oöspore were observed to contain the haploid number of chromosomes. The haploid number of chromosomes was found to be 14. However, the small size of the chromosomes made it difficult to be absolutely certain of the exact number. Meiosis, therefore, occurs during the first two divisions of the fusion nucleus.During the divisions of the nuclei just described the central body was in most cases completely digested and the wall of the oögonium became thin. The germ tube arose at a point near the thin spot in the oögonial wall, at which spot the antheridium was attached to the oögonium. The tube penetrated through the wall of the oögonium at this spot. The cytoplasm and nuclei were observed to enter the germ tube. At the time of leaving the oöspore and entering into the germ tube, the nuclei were elongated and somewhat pointed.