My view amounts to the following, viz. that after the formation of the polar cells the remainder of the germinal vesicle within the ovum (the female pronucleus) is incapable of further development without the addition of the nuclear part of the male element (spermatozoon), and that if polar cells were not formed parthenogenesis might normally occur. A strong support for this hypothesis would be afforded were it to be definitely established that a polar body is not formed in the Arthropoda and Rotifera; since the normal occurrence of parthenogenesis is confined to these two groups. It is certainly a remarkable coincidence that they are the only two groups in which polar bodies have not so far been satisfactorily observed. It is perhaps possible that the part removed in the formation of the polar cells is not absolutely essential; and this seems at first sight to follow from the fact of parthenogenesis being possible in instances where impregnation is the normal occurrence. The genuineness of the observations on this head is too long a subject to enter into here[32], but after admitting, as we probably must, that there are genuine cases of such parthenogenesis, it cannot be taken for granted without more extended observation that the occurrence of development in these rare instances may not be due to the polar cells not having been formed as usual, and that when the polar cells are formed the development without impregnation is impossible. [32] The instances quoted by Siebold, _Parthenogenesis d. Arthropoden_, are not quite satisfactory. In Hensen's case, p. 234, impregnation would have been possible if we can suppose the spermatozoa to be capable of passing into the body cavity through the open end of the uninjured oviduct; and though Oellacher's instances are more valuable, yet sufficient care seems hardly to have been taken, especially when it is not certain for what length of time spermatozoa may be able to live in the oviduct. For Oellacher's precautions, vide _Zeit. für Wiss. Zool._, Bd. xxii., p. 202. A better instance is that of a sow given by Bischoff, _Ann. Sci. Nat._, series 3, Vol. II., 1844. The unimpregnated eggs were found divided into segments, but the segments did not contain the usual nucleus, and were perhaps nothing else than the parts of an ovum in a state of disruption. Selenka found in the case of _Purpura lapillus_ that no polar body was formed in the eggs which did not develop, but in the case of Neritina, Bütschli has found that this does not hold good.