In the quartan type the red blood-corpuscles containing the parasite have a tendency to become smaller than normal corpuscles; in the tertian type they are usually larger.
In the tertian the protoplasm of the parasites is very transparent; in the quartan it is less so, and the outlines are better defined.
In the quartan the pigment is seen in the form of grains or rods of greater size than in the tertian.
Finally, the principal difference is found in the segmentation of the intracorpuscular elements. The number of spores resulting from segmentation is greater in the tertian parasite—usually more than twice as many.
That these parasitic protozoa are in truth the cause of the forms of malarial fever with which they are found to be associated can scarcely be doubted, in view of the facts that this association is a constant phenomenon, that the infected blood-corpuscles are destroyed by the parasite, that a rapid loss of red corpuscles is one of the most marked results of malarial infection, and that the parasites disappear when quinine is administered in suitable doses, thus accounting for the specific action of this drug. Finally, it has been shown by inoculation experiments that when blood containing the plasmodium is drawn from the circulation of a malarial-fever patient and injected beneath the skin of a healthy person there is a reproduction of the parasite in the blood of the inoculated individual, and, after a certain period of incubation, typical malarial paroxysms occur. Successful inoculation experiments have been made by Marchiafava and Celli, by Gualdi and Antolisei, by Bein, by Bacelli, by Di Mattel, and others.
The life-history of the malarial parasite outside of the bodies of infected individuals has not been traced. Thus far attempts to cultivate it in artificial media have failed. Nor has the presence of the plasmodium been demonstrated in water or mud taken from the marshy localities which are recognized as the source of malarial fevers, and which we therefore believe to be the normal habitat of this mischievous micro-organism. The facts relating to the seasonal and regional occurrence of malarial fevers sustain the view that they are caused by living organisms, the external development of which depends upon conditions relating to temperature, moisture, vegetable growth and decay. But among the vast number of micro-organisms of an infinite variety of species found in localities which are recognized as malarious, the little specks of protoplasm which—with a first-class oil-immersion objective, and by careful manipulation of the light—we recognize as parasitic invaders of human blood-corpuscles could scarcely be detected, and could not be differentiated from other sporelike
