The next important step was made in 1885 by Golgi, an Italian, who studied the life-history of the parasite in the blood and distinguished the three forms which cause the three most familiar kinds of malarial fevers, the tertian, the quartan and the remittent types. From this time on this parasite has been studied by physicians of many nationalities and the whole course of its life-history worked out. In order that we may understand how it was that mosquitoes were determined to be the means of disseminating this parasite we will discuss first its life-history in the human blood.
The parasites that cause the malarial fevers are Sporozoans and belong to the genus Plasmodium. Other names such as Hæmamœba and Laverania have been used for them, but the term Plasmodium is the one now most commonly employed. The three most common species are vivax, malariæ and falciparum, causing respectively the tertian, quartan and remittent fevers.
LIFE-HISTORY OF PARASITE
The life-history of all of these is very similar, the principal difference being in the length of time it takes them to sporulate. Let us begin with the parasite after it has been introduced into the blood and trace its development there. At first it is slender and rod-like in shape. It has some power of movement in the blood-plasm. Very soon it attacks one of the red blood-corpuscles and gradually pierces its way through the wall and into the corpuscle substance ([Fig. 99]); here it becomes more amœboid and continues to move about, feeding all the time on the corpuscle substance, gradually destroying the whole cell. As the parasite feeds and grows there is deposited within its body a blackish or brownish pigment known as melanin.
During the time that the parasite is feeding and growing it is also giving off waste products, as all living forms do in the process of metabolism, but as the parasite is completely inclosed in the corpuscle wall these waste products cannot escape until the wall bursts open. After about forty hours if the parasite is vivax or about sixty-five hours if it is malariæ it becomes immobile, the nucleus divides again and again and the protoplasm collects around these nuclei, forming a number of small cells or spores, as they are called. In about forty-eight or seventy-two hours, depending on whether the parasite is vivax or malariæ the wall of the corpuscle bursts and all these spores with the black pigment and the waste products that have been stored away within the cell are liberated into the blood-plasm.
Fig. 99—Diagram to illustrate the life-history of the malarial parasite. 1 is a red blood-corpuscle, 2 to 7 shows the development of the parasite in the corpuscle, a b c d and a´ b´ c´ and e the development of the parasite in the stomach of the mosquito, f g h i the development in the capsule on the outer wall of the stomach of the mosquito, k in the salivary gland.