Page:Popular Science Monthly Volume 59.djvu/207

Rh by the human parasite, while species of the genus Culex were selected by the malaria parasites of birds. The full history, finally, has been worked out in complete detail during the last two years, by Ross again, and by Grassi, in Italy, and both observers reached quite independent, but identical, results. Briefly summarizing these results, the full life history of Plasmodium malariæ may be given as follows:

The early form of the parasite, which corresponds with the sporozoites of the gregarine and of Coccidium, penetrates a red blood corpuscle, grows to adult size, and then forms spores (Fig. 3, ab). These correspond exactly with the merozoites of the Coccidia, and, like them, lead to auto-infection. At this point there is a gap in the evidence, for it is not known how long this asexual method of increase may continue; as shown above in the case of Coccidium, the sporeforming period continues for five or six days, when a period of conjugation supervenes. It may be stated here, parenthetically, that in all Protozoa, so far as known, a period of conjugation is necessary at some time during the life cycle, and without such conjugation, the organisms, which are reproduced asexually, finally decrease in size and show other signs of degeneration, ultimately resulting in death of the race (see results of Bütschli, Maupas, Hertwig, etc., upon degenerating Protozoa).

This is of considerable moment in the question of malaria, for, if the malaria-organism conforms to other Protozoa, there must come a time when this asexual sporulation will cease in any given set of individuals, and a period of conjugation must supeivene to give renewed vigor to the parasites. So far as known at the present time, this conjugation takes place only in the digestive tract of the mosquito. That it does actually take place, is undeniable from the observations of MacCallum, Ross, Grassi and others, and the conjugants again as in Coccidium, are: a small, motile, microgamete, or inale cell (one of the 'flagella' of the Polymitus form); and a larger macrogamete, or female, cell (Fig. 3, pv). Their union, observed by Ross and Grassi, takes place in the stomach of Anopheles, and the copula then makes its way into, and through, the epithelial cells lining the stomach, and finally rests against the tissue which lines the body cavity. Here it grows to a relatively large size (Fig. 3, wzz), and, when mature, its nucleus divides as in the gregarine or in Coccidium, to form a number of spores. Each of these develops a number of germs, or sporozoites, but, unlike the sporozoites of the previously described Sporozoa, these germs have no protective capsule about them, and, when the parent cyst ultimately bursts, they are liberated directly into the body cavity of the mosquito (Fig. 3, A, B). Here, in the fluids of the body cavity, they are carried to all parts of the organism, and finally reach the anterior region of the thorax, where the