Page:Popular Science Monthly Volume 59.djvu/203

Rh, in the blood cells of the human host, was correctly made out in 1888 by two Italian naturalists, Marchiafava and Celli, who showed that the young parasite, in a red blood corpuscle, is a minute granule in which no structure could be made out. The granule grows, however, at the expense of the hæmaglobin of the corpuscle, and ultimately forms spores (Fig. 3, a—f). During the life of the parent organism, the products of growth are stored up in the parasite in the form of fine granules.

 Minute germs, sporozoites (A), enter epithelial cells lining the digestive tract of a tunicate. Here they grow to a large size (B, C), ultimately breaking through a cell-membrane and falling into the lumen of the digestive tract (D). After some time in this adult condition, two individuals come together (E). The nuclei divide repeatedly (F, G), and minute gametes are ultimately formed (H, I). The gametes then fuse, two by two (I, J), forming the spores. The two nuclei also fuse (K), and the joint nucleus then divides three times in succession (L, M, N), forming right daughter-nuclei, which become the nuclei of eight germs or sporozoites (0). The sporozoites are inclosed in small calcareous capsules which, in a new host, are dissolved by the acids of the digestive fluids, thus setting free the sporozoites (A).

These, known as melanin granules, are left in the center of the parent organism when the spores are formed, but at this period the blood corpuscle, in which the sporulation occurs, disintegrates, and so liberates the spores and the melanin in the blood plasm. Like the merozoites of Coccidia, these spores make their way to new corpuscles, which they