They were carried out first by the writer alone and later in conjunction with Sir F. S. Lister. Similar results were obtained with several species of bacteria. To be able readily to alter the virulence of a bacterium outside the body without interfering with its capacity to grow on media may be of assistance in the production of antisera, in the prophylactic inoculation of individuals against certain diseases, and in the therapeutic inoculation of those already infected. An illustration may

more »

... clear what is suggested under the first and second heads. The cells of the body of an animal which has been inoculated may be likened to a company of men who are being trained to jump over hurdles. If the full height is presented straight away to the men many will be unable to clear the obstacles, and will injure themselves in trying to do so. If, however, the men are trained by being required on the first day to jump over a low obstacle and the height of this is increased on succeeding days, most if not all of them will soon succeed in -clearing the hurdles without difficulty or injury. Similarly, if a highly virulent organism is injected straight away into an .animal many cells may be injured. A gradation in the virulence of a vaccine in addition to an alteration in the quantity injected is desirable. The procedure known as piantication affords a ready means of diminishing the virulence. With regard to the third point: when vaccines are injected for therapeutic purposes it is to be recollected that the organism infecting the patient is living and is capable of adjusting itself to the increasing immune properties of the patient's blood resulting from inoculation. It may be supposed that as a result of stimulation of the bacterium in the tissues an undesirably strong focal reaction of the patient's tissues might be induced. It might therefore be of distinct advantage to begin treatment with a vaccine whose most pronounced quality was not its virulence. The patient might be aided by stimulating some other quality such as his power to produce antitoxin in order to overcome the disease from which he is suffering. This may perhaps explain the beneficial results which have followed the use of sensitised vaccines. T am. Sir. vours faithfullv.