Towards the end of the seventeenth century it occurred to one inquiring mind that a particular case of spontaneous generation, which had been accepted by everybody without hesitation, was capable of a less mysterious explanation. Francesco Redi (1626-1698), physician to the Duke of Tuscany, published in 1668 an account of his experiments on the generation of blow-flies. He found that the flesh of the same animal might yield more than one kind of fly, while the same fly might be hatched from different kinds of flesh. He saw the flies laying their eggs in flesh, and dissected eggs out of their ovaries. When he kept off the flies by gauze the flesh produced no maggots, but eggs were laid on the gauze. Redi concluded that flies are generated from eggs laid by the females. He also studied insect-galls, and the worms which feed on growing seeds. Like earlier observers, he was baffled by finding live grubs in galls or nuts which were apparently intact, and by the parasitic worms which are now and then found in the brain-case and other closed cavities of quadrupeds. Such instances led him to jump at the supposition of a "vivifying principle," which generated living things of itself—a supposition contrary to the truer doctrine which he taught elsewhere. Vallisnieri was able to explain how the egg is introduced into the rose-gall, which a little later shows no mark of injury; while Malpighi examined the young nut and found both hole and egg. How parasitic worms reach the brain-case of the sheep could be explained only in a later age. Meanwhile Swammerdam, Leeuwenhoek, Réaumur, and many other special students confirmed and extended Redi's experiments on the blow-fly; and every fresh instance of normal generation in a minute organism did something to weaken the belief in spontaneous generation.