a long period of time, either when cultivated artificially, or when growing spontaneously under natural conditions.
In this way Billroth, who has published the most recent and, in many respects, the most remarkable work upon the subject, was able to make out the whole series of changes, from the spore to the long filaments, by using for his observations the yellowish mould which formed on the wall of his laboratory, where the water leaked slowly from a loose faucet. At the same time he discovered the nature and importance of certain glistening spherical bodies frequently found in infusions containing bacteria, and called Dauersporen, or durable spores (Fig. 1, g), by Cohn, although he did not think bacteria were developed from them. Billroth demonstrated that these Dauersporen
Fig. 2.—Bacteria and Leptothrix Filaments found in a Turnip-Infusion.
form micrococci in their interior, which are set free by the bursting of the envelope, and are then capable of multiplication by scission, or of lengthening into bacteria; also, that they are endowed with great vitality, and are not destroyed by freezing, boiling, or drying. He had some which germinated after they had been kept dry for eight years; and, whenever he wished to make sure of the destruction of the spores contained in his experimental liquids, he heated them to 392° Fahr.
They are formed in the interior of bacteria, and sink to the bottom of the liquid which contains them. The importance of these facts in their bearing upon the question of spontaneous generation, and upon the innumerable repetitions and variations of the experiments with sealed flasks, which have attracted so much attention since Pasteur made them, cannot be over-estimated; for, as Prof. Wyman, of Cambridge, says, "The issue between the advocates and opponents of the doctrine in question" (spontaneous generation) "clearly turns on the extent to which it can be proved that living beings resist the action of water at a high temperature."—American Journal of Science and Art, September, 1867.
Bacteria themselves are much more easily affected by heat and cold than are these Dauersporen. Their motions cease when the temperature is reduced nearly to the freezing-point; but it may be
