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Then, one day, he began to study the way sponges and starfishes digest their food. Long before he had spied out strange cells inside these beasts, cells that were a part of their bodies, but cells that were free-lances, as it were, moving from place to place through the carcasses of which they formed a part, sticking out one part of themselves and dragging the rest of themselves after the part they had stuck out. Such were the wandering cells, which moved by flowing, exactly like that small animal, the ameba.

Metchnikoff sat down before his parlor table, and with that impatient clumsiness of a man whose hands seem unable to obey his brain, he got some little particles of carmine into the insides of the larva of a starfish. This was an ingenious and very original trick of Metchnikoff's, because these larvæ are as transparent as a good glass window; so he could see, through his lens, what went on inside the beast; and with excited delight he watched the crawling, flowing free-lance cells in this starfish ooze toward his carmine particles—and eat them up! Metchnikoff still imagined he was studying the digestion of his starfish, but strange thoughts—that had nothing to do with such a commonplace thing as digestion—little fog-wraiths of new ideas began to flutter through his head. . . .

The next day Olga took the children to the circus to see some extraordinary performing monkeys. Metchnikoff sat alone in his parlor, tugging at his biblical beard, gazing without seeing them at his bowls of starfish. Then—it was like that blinding light that bowled Paul over on his way to Damascus—in one moment, in the most fantastical, you would say impossible flash of a second, Metchnikoff changed his whole career.

"These wandering cells in the body of the larva of a starfish, these cells eat food, they gobble up carmine granules— but they must eat up microbes too! Of course—the wandering cells are what protect the starfish from microbes! Our wandering cells, the white cells of our blood—they must be what protects us from invading germs . . . they are the cause of