ANIMAL EXPERIMENTATION AND PROTEAL THERAPY 289 



about one-third as numerous, and the large monocytes dropping 

 to relative obscurity with a count of from two to four per cent. 



It would appear, then, that the physiological conditions in the 

 body of the normal mouse call for about five times as many large 

 monocytes, proportionately, as are required in the body of an 

 adult human being; and that the normal human being during 

 childhood and adolescence requires about twice as many mono- 

 cytes as are subsequently called for. 



Note now the suggestive fact that, according to the observations 

 of Cramer and Pringle, the tissue of a cancer (in mice) is of an 

 embryonic type, as shown by the fact that its cells contain only 

 about seventy per cent, of the nitrogen content of the average 

 tissues of the host. Cramer had indicated the similarity between 

 the growth of cancer and the growth of the foetus, and prelim- 

 inary experiments by Dr. Lochstad had shown that a rapidly 

 growing tissue (foetus) has a lower nitrogen value than the ma- 

 ternal organism. These experiments obviously confirm the old 

 idea that cancer tissue is of somewhat embryonic character, and. 

 suggest the possibility that chemical or other agents might be 

 found that would serve to decompound the cancer cells without 

 injuring the normal surrounding tissue. 



NATURAL IMMUNITY AND ARTIFICIAL IMMUNIZATION 



Consider now for a moment the attitude if the word be per- 

 mitted of the normal organism toward intruding cancer cells. 

 Here the evidence must come largely from the field of animal ex- 

 perimentation, but it offers suggestive hints that may be applied, 

 with due reserve, to the human subject. 



It is familiarly known that the pioneer work of Hameu and 

 Morau, expanded a little later by the work of Jensen and Leo 

 Loeb, made possible a new era of investigation by showing that 

 carcinomas and sarcomas of white mice and rats could be trans- 

 planted into other animals of the same species. It is equally well 

 known that all experimenters are agreed that a certain number 

 of individual animals show resistance to such transplantation, 

 tumors embedded in their tissues regressing and disappearing 

 "spontaneously" instead of continuing their lawless growth. 



Thus it is shown, as Dr. Isaac Levin has phrased it, that "the 

 growth of the implanted tumor depends upon the correlation be- 

 tween the virulence of the implanted cell and the resisting power 

 of the organism of the host." 



It was further shown that an artificial tumor used for inocula- 

 tion appears to acquire increasing virulence as it is passed from 

 one animal to another. But, on the other hand, it is possible to 

 induce a condition in the body of an animal which at first is sus- 



