The act of impregnation may be described as the fusion of the ovum and spermatozoon, and the most important feature in this act appears to be the fusion of a male and female nucleus; not only does this appear in the actual fusion of the two pronuclei, but it is brought into still greater prominence by the fact that the female pronucleus is a product of the nucleus of a primitive ovum, and the male pronucleus is the metamorphosed _head_ of the spermatozoon which, as stated above, contains _part_ of the nucleus of the primitive spermatic cell. The spermatic cells originate from cells indistinguishable from the primitive ova, so that the fusion which takes place is the fusion of morphologically similar parts in the two sexes. These conclusions tally very satisfactorily with the view adopted in the Introduction, that impregnation amongst the Metazoa was derived from the process of conjugation amongst the Protozoa. _Summary._ In what may probably be regarded as a normal case the following series of events accompanies the maturation and impregnation of an ovum:-- (1) Transportation of the germinal vesicle to the surface of the egg. (2) Absorption of the membrane of the germinal vesicle and metamorphosis of the germinal spot and nuclear reticulum. (3) Assumption of a spindle character by the remains of the germinal vesicle, these remains being probably in part formed from the germinal spot. (4) Entrance of one end of the spindle into a protoplasmic prominence at the surface of the egg. (5) Division of the spindle into two halves, one remaining in the egg, the other in the prominence; the prominence becoming at the same time nearly constricted off from the egg as a polar cell. (6) Formation of a second polar cell in the same manner as the first, part of the spindle still remaining in the egg. (7) Conversion of the part of the spindle remaining in the egg into a nucleus--the female pronucleus. (8) Transportation of the female pronucleus towards the centre of the egg. (9) Entrance of one spermatozoon into the egg. (10) Conversion of the head of the spermatozoon into a nucleus--the male pronucleus. (11) Appearance of radial striæ round the male pronucleus, which gradually travels towards the female pronucleus. (12) Fusion of male and female pronuclei to form the first segmentation nucleus. (76) Auerbach. _Organologische Studien_, Heft 2. Breslau, 1874. (77) Bambeke. "Recherches s. Embryologie des Batraciens." _Bull. de l'Acad. royale de Belgique_, 2me Sér., T. LXI., 1876. (78) E. van Beneden. "La Maturation de l'OEuf des Mammifères." _Bull. de l'Acad. royale de Belgique_, 2me Sér., T. XL. No. 12, 1875. (79) Idem. "Contributions à l'Histoire de la Vésicule Germinative, &c." _Bull. de l'Acad. royale de Belgique_, 2me Sér., T. XLI. No. 1, 1876. (80) O. Bütschli. _Eizelle, Zelltheilung, und Conjugation der Infusorien._ Frankfurt, 1876.