When the schizogony ceases, the merozoites, that had penetrated the epithelial cells and become trophozoites there, consist of two kinds of differently constituted individuals. One kind possesses a clear cytoplasm (fig. 67, XII), the other an opaque, richly granular cytoplasm (fig. 67, XI), while both possess a vesicular nucleus with a karyosome. In order to continue their development, the more granular individuals must be fertilized, and are therefore termed either female gametes or, on account of their size, macrogametes. The male individuals (microgametes) necessary to conjugation, are formed in greater numbers from the less dense microgametocytes or male mother-cells (fig. 67, XIId). They are slender bodies consisting chiefly of nuclear substance, and in most species bear two flagella of unequal length directed backwards, the place of insertion of which varies according to the species (fig. 67, XIIe). While the development of the microgametes is rapidly advancing a change occurs in the nucleus of the female parent forms or macrogametocytes. Parts of the karyosome are extruded (fig. 67, XIc), and the nucleus loses at the same time its vesicular form. One macrogamete results, after nuclear maturation, from one macrogametocyte. By this time the macrogametes are capable of conjugation, and the process takes place within the host, generally, however, outside the affected and degenerated host cells. The microgametes that have now become free from the very large residual body, crowd around the mature macrogametes, which often send out a small prominence (“cone of reception” or fertilization protuberance) for their reception (fig. 67, XIII). As soon as a microgamete comes in contact with this and penetrates into the cytoplasm of the macrogamete, the latter surrounds itself with a membrane which prevents the intrusion of other microgametes. The nucleus of the microgamete that has gained entry unites with the nucleus of the macrogamete, which latter is afterwards capable of forming the well-known spores. The parasite is now called an encysted zygote or oöcyst. The oöcysts of some other members of the Coccidiidea, _e.g._, _Eimeria avium_, can form their walls prior to fertilization. In such cases, a weak spot is left at one place in the cyst wall, forming a micropyle, that permits of the entry of the male, immediately after which the micropyle is closed.