Fertilization in a lichen, Basidiobolus--In Phanerogams--Here too there is reduction of the number of chromosomes by a half--'Polar cells' in lower and higher plants--Conjugation among unicellular organisms--Noctiluca--The maternal and paternal chromosomes remain apart--Actinophrys--Infusoria--Sexual differentiation of the two conjugates in Vorticella--Importance of the process of Amphimixis--Not a 'life-awakening' process--May occur independently of multiplication--The Rejuvenescence hypothesis--Pure parthenogenesis--The cycle idea--Does Amphimixis prevent natural death?--Maupas' experiments with Infusorians--Bütschli's view--Potential immortality of unicellular organisms--The immortality of unicellular organisms and of the germ-cells depends on the fact that there is no time-limit to the multiplication of the smallest living particles--Parthenogenesis is not self-fertilization--Petrunkewitsch's observations on the ova of bees--Is the chromatin really the 'hereditary substance'?--Nägeli's conclusion from the difference in size between ovum and spermatozoon--Artificial division of Infusorians--Boveri's experiments with the fertilization of pieces of ova not containing a nucleus--Fertilization gives an impulse to development even to non-nucleated pieces of ova--Merogony--The female and male nuclear substances are essentially alike--Summary. I NOW turn to the consideration of the process of fertilization in plants and unicellular organisms. With regard to plants, it can now be definitely asserted that in them, too, fertilization is essentially a conjugation of nuclei; it depends on the union of the nuclei of the two 'sex-cells.' These sex-cells are usually very small among lower plants, indeed up to the phanerogams; this is especially true of the zoosperm-like male germ-cells, but it usually holds also true of the ovum, which is but seldom burdened with an abundant supply of yolk. In spite of the many difficulties which this smallness of size puts in the way of observation, the untiring exertions of a host of excellent investigators have succeeded in following the process of fertilization in all the larger groups of plants--in algæ, fungi, mosses, ferns, and horse-tails among cryptogams, and in phanerogams.