Megasporogenesis, embryo-sac formation and endosperm and embryo development were studied in a tetraploid variety of watermelon which was artificially obtained some years ago at the Instituto Agronômico de Campinas, State of São Paulo. Since the first stages of embryo-sac formation the avules are of different sizes. In the "larges" ovules, megasporogenesis occurs before the opening of the flowers; almost all the ovules show complete embryo-sac at the day of anthesis and fertilization takes place 24 to 48 hours afterwards. Egg cell begins to divide about the seventh day; at this time the endosperm has about 16 nuclei. Cholozol endosperm houstorium has been detected and its maximum length is shown in ten-day ovules, in which the embryo is at a globular stage. In the ovules of 13 and 14 days it hos been observed well developed endosperm with an atrophied haustorium when present; the embryo was then heart-shaped. Completely mature seeds were found in unripe fruits 20 and 22 days old; in such seeds the embryo occupies the whole inner space before being filled by the endosperm and the nucellus; the latter is reduced to o thin and transparent envelop of the embryo; externally there is the hard and thick outer integument. A relation between the embryo-sac development and nucellar growth hos been well established. It was noticed a very large elimination of ovules during fruit formation either before or after fertilization: the ovules became more and more delayed in the development of their different parts and eventually formed a large series of "aborted" seeds in the ripe fruit. Only a small port of the ovules became normal seeds thus explaining why this tetroploid variety of watermelon shows a much smaller number of seeds than the original diploid.