The remarkable observations of Greeff (No. 88) on the parthenogenetic development of the eggs of _Asterias rubens_ tell, however, very strongly against the above hypothesis. Greeff has found that under normal circumstances the eggs of this species of starfish will develop without impregnation in simple sea-water. The development is quite regular and normal, though much slower than in the case of impregnated eggs. It is not definitely stated that polar cells are formed, but there can be no doubt that this is implied. Greeff's account is so precise and circumstantial that it is not easy to believe that any error can have crept in; but neither Hertwig nor Fol have been able to repeat his experiments, and we may be permitted to wait for further confirmation before absolutely accepting them. To the suggestion already made with reference to the function of the polar cells, I will venture to add the further one, _that the function of forming polar cells has been acquired by the ovum for the express purpose of preventing parthenogenesis_. The explanation given by Mr Darwin of the evil effects of self-fertilization, viz. the want of sufficient differentiation in the sexual elements[33], would apply with far greater force to cases of parthenogenesis. [33] Darwin, _Cross- and Self-Fertilization of Plants_, p. 443. In the production of fresh individuals, two circumstances are obviously favourable to the species. (1) That the maximum number possible of fresh individuals should be produced, (2) That the individuals should be as vigorous as possible. Sexual differentiation (even in hermaphrodites) is clearly very inimical to the production of the maximum number of individuals. There can be little doubt that the ovum is potentially capable of developing _by itself_ into a fresh individual, and therefore, unless _the absence_ of sexual differentiation was very injurious to the vigour of the progeny, parthenogenesis would most certainly be a very constant occurrence; and, on the analogy of the arrangements in plants to prevent self-fertilization, we might expect to find some contrivance both in animals and in plants to prevent the ovum developing by itself without fertilization. If my view about the polar cells is correct, the formation of these bodies functions as such a contrivance.