procured a number of infected sparrows and let loose upon them a number of mosquitoes of a species belonging to the genus _culex_. These mosquitoes, after from one to ten days, he dissected and examined their stomachs. He found in the stomach-wall pigmented bodies exactly similar to those which he found in the stomach-walls of mosquitoes fed on human malarial blood. He found that they increased in size and in a week or ten days grew from six to eighty micro-millimetres in diameter. When they became of considerable size they protruded like warts from the surface of the insect’s stomach and were included in a very definite capsule. At this stage the capsule was filled with a vast number of very minute rod-like bodies. These capsules, which now projected into the body cavity of the insect, being over-distended, ruptured and discharged the rod-like bodies into the body cavity of the mosquito. For a time Ross could get no further than this. He could not find what became of the rod-like bodies. One day, in dissecting the head of a mosquito, he encountered two small trilobed glands the ducts from which united to form a main duct. The glands lay on either side of the head and the common duct he traced to the base of the proboscis of the mosquito. This was the salivary gland of the mosquito. He found that the cells of the gland contained rod-like bodies exactly like those which he had found inside the parasitic capsules in the stomach-wall. He concluded that somehow these ‘germinal rods’ (for so he called them) had managed to find their way into the salivary gland of the mosquito. It immediately occurred to him that this might be the route by which the parasite escaped from the mosquito into its vertebrate host. No sooner had the idea occurred to Ross than he put it to the test of experiment. He selected a number of sparrows in whose blood he satisfied himself that there were no parasites and let loose upon them a number of mosquitoes which he had already infected with malarial parasites. He found after a week or ten days that the sparrows which were experimented upon sickened and many of them died; and in their blood he found the malarial parasite. We now understand why the flagellated body is developed outside the human host: because its function lies outside the human host. We now understand why the flagella break away and enter the granular sphere: they impregnate it and start it on the road of development. We now understand why MacCallum’s vermicule is beaked and endowed with powers of locomotion and penetration: that it may approach and penetrate the stomach of the mosquito. And we now know why the sporozooites, the ‘germinal rods,’ enter the mosquito’s salivary gland: that they may be injected into vertebrate issue and so pass the parasite from vertebrate to vertebrate.