I need not discuss the whole of this long process of scientific development, and have only touched upon it because I wished to emphasize that the conception of the process of fertilization was for a long time quite erroneous, and has only attained to clearness in recent times. Pairing as it is seen in the higher animals was for long regarded as the essential part of the process, and a mysterious life-awakening influence was assumed in regard to it; and even when it was understood that not the copulation, but the union of two living units which was always brought about thereby--the union of the male and the female germ-cells--was the essence of 'fertilization,' this was still regarded as a life-awakening process, and the way to a true understanding of the facts was thus once more blocked. The simplest form of sexual reproduction in many-celled animals is found, among others, in the Volvocineæ, those green, spherical, freshwater cell-colonies which we have already studied in relation to reproduction by asexual germ-cells. Among them it is the rule that, after a long series of generations producing only 'asexual' germ-cells, colonies occur in which each germ-cell is no longer able to develop a new colony alone, but can do so only after it has united with another germ-cell. Now, as we have seen, there are Volvocineæ in which the differentiation of cells into those of the body (soma) and those concerned with reproduction has not been established, and all the cells are therefore alike. In these, as for instance in the genus _Pandorina_ (Fig. 62, p. 257), when sexual reproduction is to occur the whole colony breaks up into sixteen cells; these burst forth from the gelatinous matrix in which they have been hitherto enclosed, swim about in the water with the help of their two flagella, meet other similar free-swimming cells and conjugate with these. The two swimming cells come close to each other, draw in their flagella, sink to the ground in consequence, and fuse completely both as to the cell-body and the nucleus. They assume a spherical form, lose the eye-spot, become surrounded with a tough cell-skin or cyst, and so remain for a longer or shorter time as so-called 'zygotes' or lasting spores. Then they develop by repeated cell-division into one of the sixteen-celled _Pandorina_ colonies with which we are already familiar; this bursts forth from the capsule and swims freely about in the water again.