_Stephanosphaera_ has its eight cells spindle shaped, and lying along equidistant meridians of its sphere; in vegetative reproduction each of these breaks up in its place to form a young colony, and the eight daughter-colonies are then freed. In conjugation, each cell of the colony breaks up into broods of 4, 8, 16, or 32 small gametes, which swim about within the general envelope, and pair and fuse two and two: this is "isogamous," "endogamous" conjugation. In _Pandorina_ (Fig. 45) the cells are rounded, and are from 16 to 32 in each colony. The vegetative reproduction in this, as in _Eudorina_, is essentially the same as in _Stephanosphaera_. In conjugation the cells are set free, and are of three sizes in different colonies, small (S), medium (M), and large (L). The following fusions may occur: S × S, S × M, S × L, M × M, M × L. Thus the large are always female, as it were, the medium may play the part of male to the large, female to the small; the small are males to the medium and to the large. The medium {129}and small are capable, each with its like, of equal, undifferentiated conjugation; so that we have a differentiation of sex far other than that of ordinary, binary sex. _Eudorina_, however, has attained to "binary sex," for the female cells are the ordinary vegetative cells, at most a little enlarged, and the male cells are formed by ordinary cells producing a large flat colony of sixty-four minute males or sperms. In some cases four cells at the apex of a colony are spermogonia, producing each a brood of sperms, while the rest are the oospheres. The transition to _Volvox_ must have arisen through the sterilisation of the majority of cells of a colony for the better nutrition of the few that are destined alone for reproduction. [Illustration: FIG. 45.—_Pandorina morum._ A, entire colony; B, asexual reproduction, each zooid dividing into a daughter-colony; C, liberation of gametes; D-F, three stages in conjugation of gametes; G, zygote; H-K, development of zygote into a new colony. (From Parker's _Biology_, after Goebel.)]