Of the earlier observations on this subject there need perhaps only be cited one of E. van Beneden, on the rabbit's ovum, shewing the presence of two nuclei before the commencement of segmentation. Bütschli was the earliest to state from observations on _Rhabditis dolichura_ that the first segmentation nucleus arose from the fusion of two nuclei, and this was subsequently shewn with greater detail for _Ascaris nigrovenosa_, by Auerbach (76). Neither of these authors gave at the first the correct interpretation of their results. At a later period Bütschli (80) arrived at the conclusion that in a large number of instances (_Lymnæus_, _Nephelis_, _Cucullanus_, &c.), the nucleus in question was formed by the fusion of two or more nuclei, and Strasburger at first made a similar statement for _Phallusia_, though he has since withdrawn it. Though Bütschli's statements depend, as it seems, upon a false interpretation of appearances, he nevertheless arrived at a correct view with reference to what occurs in impregnation. Van Beneden (78) described in the rabbit the formation of the original segmentation nucleus from two nuclei, one peripheral and the other central, and deduced from his observations that the peripheral nucleus was derived from the spermatic element. It was reserved for Oscar Hertwig (89) to describe in _Echinus lividus_ the entrance of a spermatozoon into the egg and the formation from it of the male pronucleus. The general fact that impregnation consists in the fusion of the spermatozoon and ovum has now been established for some forms in the majority of invertebrate groups (Arthropoda and Rotifera excepted). Amongst Vertebrata also it has been shewn by E. van Beneden that the first segmentation nucleus is formed by the coalescence of the male and female pronucleus. Calberla, and Kupffer and Benecke have demonstrated that a single spermatozoon enters at first the ovum of Petromyzon. The contact of the spermatozoon with the egg membrane causes in Petromyzon active movements of the protoplasm of the ovum, and a retreat of the protoplasm from the membrane. In Amphibia the appearance of a peculiar pigmented streak extending inwards from the surface of the pigmented pole of the ovum, and containing in a clear space at its inner extremity a nucleus, has been demonstrated as the result of impregnation by Bambeke (77) and Hertwig (90). There can be little doubt that this nucleus is the male pronucleus, and that the pigmented streak indicates its path inwards. Close to it Hertwig has shewn that another nucleus is to be found, the female pronucleus, and that eventually the two join together. In Amphibia the phenomena accompanying impregnation are clearly of the same nature as in the Invertebrata. A precisely similar series of phenomena to those in Amphibia has been shewn by Salensky to take place in the Sturgeon.