The oöcysts usually reach the duodenum unharmed, with food or drink. Under the influence of the powerful digestive juices (especially the pancreatic) now encountered, the oöcysts soften, as do the sporocysts, and ultimately two sporozoites emerge from each sporocyst. The sporozoites are from 7 µ to 10 µ long, and each is vermicular with a uniform nucleus (fig. 69, A). After a short period of active movement in the gut, each sporozoite penetrates an epithelial cell (figs. 68 _spz_, 69, B), and once within, gradually becomes rounded (fig. 69, B, C). It grows rapidly, feeding on the contents of the host cell and living as a trophozoite (fig. 69, _D_). When the parasite is from 10 µ to 12 µ in diameter, usually multiplication by schizogony (fig. 69, E-H) begins. The nucleus of the parent cell, now called a schizont, divides into a number of portions that become arranged at the periphery (fig. 69, E). Cytoplasm collects around each nucleus (fig. 69, E, F) and gradually a group of daughter individuals (merozoites) is produced (fig. 69, G), the nucleus of each merozoite showing a karyosome. [Illustration: FIG. 68.--Small piece of epithelial lining of gut of heavily infected Grouse chick, showing various stages in life history of the parasite _Eimeria avium_; _par_, parasite (trophozoite); _mz_, merozoite; _sch_, schizont; _spz_, sporozoite; _ooc_, oöcyst; ♂, male gametocyte; ♀, female gametocyte. × 750. (After Fantham.)] The merozoites of _Eimeria avium_ are arranged “en barillet,” like the segments of an orange (figs. 68 _mz_, 69, G), therein differing from those of _E. schubergi_, which are arranged “en rosace.” They separate from one another (fig. 69, H), penetrate other epithelial cells, where they may, in turn, become schizonts. Eight to fourteen merozoites are usually formed by each schizont, twenty have been found, while in cases of intense infection when space has become limited, the number may be only four. After a number of generations of merozoites have been formed, a limit is reached both to the multiplicative capacity of the parasite and to the power of the bird to provide the invader with food. Consequently, resistant forms of the parasite are necessary, and the trophozoites begin to show sexual differentiation instead of forming schizonts, that is, gametogony commences. [Illustration: FIG. 69.--_Eimeria avium_. Diagram of life-cycle. For explanation see text. (After Fantham.)]