THE CELLS AS PAEASITES 333 



conditions we observe that there is an interaction of two forces : 

 the invasive properties of these cells, the protective properties of 

 the surrounding maternal tissues, by which interaction the extent 

 of invasion of the cells is strictly limited to the placental site. 



3. The development of the syncytioma malignum must there- 

 fore be attributed to either an increase in the invasive properties 

 of the syncytial cells, or a lessened resistance on the part of the 

 maternal tissues, or a combination of the two. 



4. If micro-parasites play any part in the production of the 

 tumour, that must be either by exalting the infiltrative powers 

 of the one or by lowering the resistant powers of the other. 



5. Proof positive of the existence of such specific micro-para- 

 sites is still wanting, and in its absence it is difficult to conceive 

 how specific micro-parasites should bring about these results. 



6. Admitting that micro-parasites and their products are cap- 

 able of inducing cell proliferation, we must also admit that other 

 external agencies have the same capacity, so that micro-para- 

 sites, at most, may be one of the causes of aberrant cell growth. 



7. The prevalent conception of cancer parasites, as existing 

 within the cancer cells, involves the idea of malignant growths 

 being the product of parasites acting within parasites ; for, as 

 shown by this study of the syncytioma, the tumour cells them- 

 selves are essentially parasitic in the organism. 



In short, tracing the present popular idea to its legitimate 

 conclusion, the most that can be said for it is, as the association 

 of parasitic ideas may already have reminded some of my 

 audience, that it is in harmony with the great generalization of 

 the plagiaristic poet, that 



Great fleas have little fleas upon their backs to bite 'em, 

 And little fleas have lesser fleas, and so ad infinitum. 



Is this generalization also to be applied to explain the action 

 of pathogenic bacteria ? Have they their stimulant parasites ? 



I am willing to see, in the long-continued action of micro- 

 parasites, a process which may, like other modes of stimulation, 

 initiate aberrant and neoplastic cell growth, but beyond this 

 point I cannot advance. We seem to be asked to contemplate 

 a most extraordinary state of affairs, so that I cannot accept 

 the view that micro-parasites are in constant action in malignant 

 growths, and are essential for the extension of the same. 



