B. MULTICELLULAR INDIVIDUALS. The most important genera are _Gonium_, _Stephanosphæra_, _Pandorina_, _Eudorina_, _Volvox_.--_Gonium_ has 4 or 16 cells arranged in a definite pattern in a flat plate (Fig. 44). _Pandorina_ (Fig. 45), has 16 cells arranged in a sphere (Fig. 45 _A_). The vegetative reproduction takes place in this way: each cell, after having rounded off, and after the withdrawal of the cilia, divides itself into 16 new ones (Fig. 45 _B_), each forming a new individual, which soon grows to the size of the mother-individual. It was in this Alga that the conjugation of self-motile gametes was first discovered by Pringsheim, 1869. When conjugation is about to take place, each cell divides into sixteen, as in vegetative reproduction, but the 16 × 16 cells all separate from one another (Fig. 45 _C_, female gametes, and _D_, male gametes), and swarm solitarily in the water. The male are, most frequently, smaller than the female, but otherwise they are exactly alike; they are more or less pear-shaped, with a colourless anterior end, 2 cilia, a red “eye-spot,” etc. After swarming for some time they approach each other, two and two, generally a large and a smaller one, and come into contact at their colourless end; in a few moments they coalesce and become one cell (Fig. 45 _E_, _F_), this has at first a large colourless anterior end, 4 cilia, and 2 “eye-spots” (Fig. 45 _G_), but these soon disappear and the cell becomes uniformly dark-green and spherical, and surrounds itself with a thick cell-wall, losing at the same time its power of motion: the zygote (Fig. 45 _H_) is formed, and becomes later on a deep red colour. On the germination of the zygote, the protoplasmic cell-contents burst open the wall (Fig. 45 _J_), and emerge as a large swarmspore (Fig. 45 _K_) which divides into 16 cells, and the first small individual is formed (Fig. 45 _L_, _M_). _Eudorina_ is like _Pandorina_ in structure, but stands somewhat higher, since the contrast between the conjugating sexual cells is greater, the female one being a motionless oosphere. [Illustration: FIG. 46.--_Volvox globator_, sexual individual: _a_ antheridia which have formed spermatozoids; _b_ oogonia.]