The life cycle of _Nosema apis_, parasitic in bees, may be taken as an example of that of a microsporidian. The infection of the host is initiated by the ingestion of spores of _N. apis_ in food or drink contaminated with the excrement of other infected bees. Under the influence of the digestive juice of the bee the spore-coat (sporocyst) softens, the polar filament is ejected and anchors the spore to the gut epithelium, and the minute amœbula contained in the spore emerges. The amœbula is capable of active amœboid movements (fig. 98, _b_) and so is termed the planont or wandering form (fig. 98, _a_). After a short time each planont penetrates between or into the cells of the epithelium of the gut, a few only passing through into the body cavity. Within the cells the amœbulæ become more or less rounded, lose their power of movement, and after a period of growth of the trophozoite (fig. 98, _c_) commence to divide actively, these dividing forms being known as meronts (fig. 98, _d_). Various forms of fission occur, and during this phase, termed merogony, the numbers of the parasite within the host are greatly increased, with concomitant destruction of the epithelium (fig. 98, _e_). After a time sporogony commences. The full-grown meront becomes successively the pansporoblast and sporoblast. Nuclear multiplication and differentiation ensue and five nuclei are ultimately produced. At the same time a sporocyst is secreted, and two vacuoles are produced within. One is the polar capsule, and within it the polar filament is differentiated; the other forms the posterior vacuole (fig. 98, _g_). Between the two vacuoles the body cytoplasm or sporoplasm forms a girdle-like mass. Of the nuclei, one regulates the polar capsule, two control the secretion of the sporocyst, and two remain in the sporoplasm. The polar capsule and polar filament are not usually visible in the fresh condition, but can be demonstrated by the use of various chemical reagents (fig. 100). The sporoplasm ultimately becomes the amœbula (fig. 98, _g_) which issues from the spore after the ejection of the polar filament. [Illustration: FIG. 99.--_a_, section through the abdominal wall of a silkworm, whose epithelial cells contain Microsporidia (_Nosema bombycis_); _b_, a spore, the contents of which are escaping. (After Balbiani.)] [Illustration: FIG. 100.--_Nosema bombycis_, Naeg. Spores treated with nitric acid, thus rendering the polar capsule perceptible, and the filament has protruded from one of the spores. (After Thélohan.)]