66 



of the vertebrate host are still contained in the shell of the 

 red corpuscles. In some species they present a special 

 crescentic form. They circulate in the blood for some days, 

 or even weeks, according to the species, w^ithout change ; 

 but when they are drawn into the alimentary canal of cer- 

 tain mosquitoes they undergo further development. 



Very shortly they break from the enclosing corpuscles, 

 swell up slightly, and then become, some of them male 

 gametocytes and some of them female gametocytes. The 

 male gametocytes emit a variable number of microgametes 

 which escape into the serum of the ingested blood. The 

 individual microgametes are delicate motile filaments, 

 sometimes with a slight swelling in the middle. The female 

 gametocyte gives rise to a single motionless macrogamete. 



One microgamete now passes bodily into a macrogamete 

 and fertilises it, fusing with it, and producing a zygote. 

 This possesses the power of movement, and works its way 

 through the wall of the stomach, or middle intestine, and 

 fixes itself to the outside of this organ. It grows rapidly, 

 acquires a thick cell wall, and protrudes into the body-cavity 

 of the mosquito. The nucleus divides into a number of 

 portions which become spherical, and each of these 

 bears on its external surface a large number of delicate 

 filaments, each affixed to it by one extremity. When 

 mature the cell wall bursts and the filaments are liberated. 

 Each is a delicate flagellula, bearing two opposite tapering 

 flagella. These are carried by the blood-stream into all 

 parts of the insect, and they especially accumulate in very 

 large numbers in the cells of the salivary glands, from which 

 they pass into the duct which runs to the extremity of the 

 middle stylet, and thence escape when the insect bites, into 

 the blood of a new vertebrate host. When a mosquito 

 attacks a vertebrate animal it punctures the skin, and 

 injects a small quantity of a fluid secreted by its salivary 

 glands. This is said to prevent coagulation of the blood, 

 and to increase its fluidity as it is drawn through the 

 insect's suctorial apparatus. Having thus gained admission 

 to a vertebrate host, the flagellulse attack the corpuscles, 

 and become the intra-corpuscular amoebulse with which we 

 started. 



Another interesting and important series of experiments 

 was carried out by Grassi on the subject of the hereditary 

 transmission of the disease among the mosquitoes them- 

 selves. These all led to negative results. Specimens of 

 A. claviger were bred from parents taken in malarial houses, 



