The phyletic origin of germ-cells obviously coincides with the differentiation of the first multicellular organisms by division of labour[133]. If we desire to investigate the relation between germ-cells and somatic cells, we must not only consider the highly developed and strongly differentiated multicellular organisms, but we must also turn our attention to those simpler forms in which phyletic transitions are represented. In addition to solitary unicellular organisms, we know of others living in colonies of which the constituent units or cells (each of them equivalent to a unicellular organism) are morphologically and physiologically identical. Each unit feeds, moves, and under certain circumstances is capable of reproducing itself, and of thus forming a new colony by repeated division. The genus _Pandorina_ (Fig. I), belonging to the natural order _Volvocineae_, represents such ‘homoplastid’ (Götte) organisms. It forms a spherical colony composed of ciliated cells, all of which are exactly alike: they are embedded in a colourless gelatinous mass. Each cell contains chlorophyll, and possesses a red eye-spot, and a pulsating vacuole. These colonies are propagated by the sexual and asexual (Fig. II) methods alternately, although in the former case the conjugating swarm-cells cannot be distinguished with certainty as male or female. In both kinds of reproduction, each cell in the colony acts as a reproductive cell; in fact, it behaves exactly like a unicellular organism. [Illustration: I. _Pandorina morum_ (after Pringsheim), a swarming colony. II. A colony divided into sixteen daughter colonies: all the cells alike. III. A young individual of _Volvox minor_ (after Stein), still enclosed in the wall of the cell from which it has been parthenogenetically produced. The constituent cells are divided into somatic (_sz_), germ-cells (_kz_).]