are ready to form spores. The nucleus divides repeatedly, and a great number of buds are formed around the daughter nuclei (Fig. 2, b, c). These buds elongate from the periphery of the parent organism and radiate from it, like the spines of a sea urchin. When fully developed, the spores, or, as they are technically known, the merozoites, drop off the parent cell and work their way through the fluids of the digestive tract until they come to the cells lining it, and then, like the sporozoites, they penetrate the cells, grow at their expense, and again reproduce spores as before (Fig 2, a to c). This process thus tends to spread the disease among the cells of the digestive tract in the one host, and it will be observed that the reproductive process is not accompanied by the union of two gametes, as in the case of Monocystis. Coccidium is thus distinguished from the latter in having a method of asexual multiplication leading to auto-infection. This process, however, cannot continue indefinitely, and, after five or six days, a method of sexual multiplication supervenes. The preliminary stages of this process do not differ from the formation of the merozoites, and similar buds are formed which break off' and penetrate the epithelial cells as before. The further history, however, differs markedly from that of the merozoite. Some of the resulting parasites give rise to immense numbers of minute, active, thread-like buds, the microgametes, which radiate from the parent cell like the merozoites (hj). Others do not form buds at all, but merely enlarge until they are as large, or larger than, the ordinary full-grown parasites (df). One of the small forms then fuses with a large form, in conjunction; and the result, or copula, secretes a firm cyst about itself, and then divides into spores (2, g). Each spore then secretes about itself a second coating which becomes impregnated with calcareous matter, and, within this cyst, the cell divides into a small n,umber of sporozoites (k). In this condition the primary cysts are emptied to the outside, where they are ultimately taken up by some new host in whose digestive tract the cysts are dissolved and the sporozoites liberated to renew the cycle (Fig. 2, l).
It thus appears that, in Coccidium, the life cycle is more complicated than in the gregarine, in having a period of asexual reproduction by which auto-infection is accomplished, alternating with a period of sexual multiplication during which the parasite is carried from one host to another. Coccidium differs further from Monocystis in that the conjugating gametes are sexually differentiated, the small, active one, or microgamete, functions as the male cell, and the larger, quiescent one, or macrogamete, as the female or egg cell, while in the gregarine, on the other hand, the conjugating gametes are of equal size.
We may now consider the somewhat more complicated life cycle of the malaria organism. The process of spore-formation of this para-
