Leeuwenhoek was a pure observer of facts and made no attempt at speculation, but his discoveries soon started the theorists to discussing the origin of these minute organisms. Most observers, as was probably to be expected, believed that they arose spontaneously. Needham, in 1749, described the development of microörganisms around grains of barley in water. Bonnet, in 1768, suggested that probably Needham’s animalcules came from ova in the liquid. The Abbot Spallanzani, in 1769, called attention to the crudeness of Needham’s methods and later, in 1776, attempted to disprove spontaneous origin by heating infusions of organic material in flasks and then _sealing_ them. His critics raised the objections that heating the liquids destroyed their ability to support life, and that sealing prevented the access of fresh air which was also necessary. The first objection was disproved by the accidental cracking of some of the flasks which thereafter showed an abundant growth. This accident seemed also to support the second objection, and Spallanzani did not answer it. Though Spallanzani’s experiments failed to convince his opponents, they led to important practical results, since François Appert, in 1810, applied them to the preserving of fruits, meats, etc., and in a sense started the modern canning industry. [Illustration: FIG. 2.--The first drawings of bacteria by Leeuwenhoek. The dotted line _C-D_ indicates the movement of the organism.] [Illustration: FIG. 3.--Schultze’s experiment. The set of bulbs next to the face contained KOH and the other set concentrated H₂SO₄. Air was drawn through at frequent intervals from May until August but no growth developed in the boiled infusion.]