Several papers have been written on the development of the peanut seed, but none described the process from the beginning until the mature seed with enough detail; also no one described the transformations of the seed parts and related them with age or size. The present observations began with ten-day old ovaries; ovaries older than twenty days were collected and studied according to their sizes. Measurements were taken from the length and width of both the ovule and embryo-sac, and also of the width of the integuments. The number of endosperm nuclei and the number of cells in the different integument layers was counted. The peanut ovule is normal and has two integuments. The embryo-sac contains one embryo and the nuclei of the endosperm. No nucellar cells are found in ten-day old ovules. The endosperm is of the nuclear type, having a large number of nuclei before the cellular walls beguin to form. The cellular walls progress from the periphery of the sac towards its center, and from the micropylar to the chalazal region. The nuclear division in the endosperm is very intense; the nuclei of the cellular endosperm are a little smaller than the free nuclei. The endosperm disappears when the seed is physiologically mature. The embryo has two cotyledones; they grow to form the body of the seed; the cellular contents in the mature seed are chiefly oil. Increase in size of the seed is due to the growth of the integuments; this growth makes the volume of the embryonic cavity larger, which is rapidly filled by the cotyledones.