MICRO-OKGANISMS AS PARASITES. 27 



The line of enquiry which has led up to the discovery of the 

 function of leucocytes as destroyers and devourers of foreign 

 particles, injurious or unnecessary to the body, was begun by 

 Haeckel, who watched the corpuscles of a naked sea-snail eating 

 up particles of indigo, in amoeba fashion. He subsequently 

 watched the same process in the colourless blood of other inverte- 

 brates. Many other observers continued this line of investigation, 

 the most eminent and successful being the Russian scientist, 

 Metschnikoff. He found that the white cells of numerous 

 invertebrate animals devoured finely divided particles of carmine, 

 also disintegrated bodies of entomostraca, human blood-corpuscles, 

 milk globules, starch granules, etc. But more extraordinary and 

 important discoveries were to come. I will give in detail his 

 experiments on DapJmia (the water-flea). He kept many of these 

 little creatures in a tank, and after a time he found them infected 

 with fungus spores, which germinated and were dispersed by the 

 blood-current over the body. They were deposited in those parts 

 where the blood-current is slowest, and in these places (the 

 cephalic and hinder portions of the mantle cavity) heaps of 

 conidia collected. In the meantime the leucocytes did not 

 remain idle during the invasion ; they attacked and devoured the 

 conidia, took them into their interior, and digested them. If a 

 conidium were too much for one cell, others joined it, and 

 fused to form a giant cell or plasmodium, the better to exterminate 

 the invader. Notice this especially, for farther experiments have 

 shown that the same rule holds good for every organism, whether 

 animal or vegetable. " If the leucocytes finally overpower the 

 spores, the Daphnia lives ; if not, the spores over-run the 

 crustacean, and death is the result.'"' 



In the B. Medical Journal^ a house-surgeon described his 

 experience with one of the fresh-water Algae, C/iara, which with 

 other pond weeds he kept in a glass jar placed in a hospital ward. 

 Soon he saw the Chara begin to drop, and the other water-plants 

 died outright. After two or three days the Chara began to revive, 

 and resumed its bright-green colour. The surgeon then examined 

 the water of the jar, and the tissues of Chara. He found, as 

 usual, in water kept in a hospital ward, that it was full of septic 

 organisms. The dead plants swarmed with living bacteria, but 



