Vaucheria geminata is found most abundantly in the spring and autumn. Sexual reproductive organs are most abundant in Sept. and Oct.; they are also found about April 15. There are 1 or 2 periods of general mitosis in the oogonium. The egg nucleus undergoes mitosis just in advance of the vegetative nuclei. The nuclear membrane disappears in both the vegetative and reproductive nuclei during mitosis. Spindle fibers are visible in the vegetative nuclei in the metaphase. The cross wall begins to form about the time the sperm enters the oogonium, and is of vacuolar origin. The vegetative nuclei begin disintegration at the time of fertilization. There is no migration of nuclei either to or from the oogonium. A swelling forms from the oogonium in the "receptive region," which comes in contact with the antheridium at the time of fertilization; the [male] element enters through this process. Fertilization occurs during the latter part of the night. Maturation divisions in the egg nucleus occur in the afternoon. The formation of the cross wall, the disintegration of the vegetative nuclei, and the fusion of the gamete nuclei occur in the latter part of the night and early morning. The number of vegetative nuclei in the oogonium varies in specimens examined from 55 to 141. They are increased considerably in number during the periods of mitosis. The fusion of the gamete nuclei does not occur immediately when the [male] element enters the oogonium. The [male] nucleus lies beside the egg nucleus and there enlarges to approximately the size of the [female] gamete nucleus. In the center of the oogonium a vacuolar structure forms after fertilization. The zygote nucleus takes its position in the midst of this structure.