The theory of planetary accumulation leads quite definitely to the conclusion that the formation of Oort's cometary cloud is the result of ejection of bodies to the outermost parts of the solar system due to encounters with the giant planets during their growth. Uranus and Neptune could have grown to their present dimensions only if the initial mass of solid material in their zones were substantially larger than that of these planets. The relative velocities of the bodies were increased through perturbations by the planetary embryos, and on reaching the escape velocity they would start to be ejected. Our concept of this process differs from that suggested by Öpik by the assumption that Jupiter and Saturn accreted hydrogen, not in solid but in gaseous state, and by the introduction of a more effective mechanism for the interaction with several embryos. In their final stages the embryos ejected amounts of mass an order of magnitude higher than the amounts accreted. Most of the mass was ejected into interstellar space by Jupiter, but the cometary cloud was created mainly by Neptune. The mass of the cloud is estimated to be about three times that of the Earth.