The essence of Dr. Heidenhain's experimental work is the production of neoplasms in mice by the injection of extracts of cancer tissue. The rest of the book also contains elaborate theoretical discussion of the implication of these experiments and their relation to other theories of the production of cancer. He believes that there is a carcinogenic agent in cancer tissue that has the power to cause the formation of secondary tumors on its injection into animals of different species. The work is excellently illustrated with superb photomicrographs of cancers produced experimentally in mice.
There are, however, insuperable difficulties to the acceptance of this theory. The well known frequency of spontaneous disease in mice is the greatest of these. Sixteen per cent of the animals into which injections were made developed tumors, which is a scarcely greater ratio than one would normally expect in many strains of laboratory mice. There is