The mechanism underlying the multiplication of cells has naturally held a dominant place in the mind and work of the biologist since the beginning of the microscopic era. Coupled with it has always been the more intricate problem of cell differentiation in the community cell life of an animal or plant body. If the mechanisms concerned in these two processes can be isolated and placed at the disposal of the biologist, many of the difficulties encountered today with the acute infections, the anemias and the tumors of the body may be overcome.
Practically all diseases caused by living parasites are associated with altered characteristics or diminished or increased rate of multiplication of some of the specific component cells of the body. For example, the organisms of many of the acute infections possess a stimulus to the multiplication of the polymorphonuclear granular leukocytes. Perhaps the stimulus is a common factor to