The minute bodies discovered in the vaccine lymph of calves by Van der Loeff still continues to puzzle investigators. Reasons for this are the repeated failures to find, even by improved methods of bacteriologic research, any specific micro-organism as a cause for variola and the difficulty in accounting for the vaccine bodies except as parasites. Foremost among those works maintaining their intrinsic somatic origin is that of Hückel ascribing them to a specific degeneration of the epithelial cells; prominent among the supporters of their parasitic nature is Guarnieri, who concluded, from a study of their morphology as they occur in the vesicles produced by vaccinating rabbits' corneas, that the vaccine bodies are protozoa.
Soon after the work of Guarnieri, L. Pfeiffer found them in the blood of both variolous and vaccinated people as free, swimming, nucleated bodies, one-fourth the size of a red corpuscle. These observations were soon confirmed by