170 CROWN-GALL OF PLANTS. 



of the nonparasitic origin of the tumor, either no parasites being 

 introduced or so few and these so reduced in vitality by long presence 

 in the tumor or by exposure to the juices of the crushed cells, which 

 we may suppose to be more or less germicidal, that they are overcome 

 and destroyed by the normal activities of the body. It is possible 

 also that there may be some special mechanism of infection. Here 

 might also be pointed out that most of this evidence has been derived 

 fi'om mouse tumors, and that we are under no obligation to consider 

 all malignant tumors as etiologically identical. 



(3) A parasite might be present and not isolated because unable 

 to grow on the media commonly offered to it, as in the case of syphilis 

 and yaws. The most striking evidence of this nature the senior 

 writer has had brought to his attention was the failure of a strepto- 

 coccus associated with endocarditis to grow on media obtained fi'om 

 one of the best human pathological laboratories in the country, but 

 which grew readily in slightly different media. The gi'owth of the 

 organism, as was afterwards determined by him, was inhibited by 

 the presence of too much sodium hydroxide in the bouillon. This 

 particular organism he also observed to be very sensitive to sodium 

 chloride, so that a slight excess of sodium chloride in the agar or 

 bouillon would also inhibit growth. Had only one bouillon or agar 

 been used the experiment would have failed. This organism was 

 isolated in + 15 agar and bouillon, but would not grow in zero bouillon 

 or agar (October, 1906). 



In the light of these facts there can be little doubt that many of 

 the blood tests in arthritis and endocarditis which have been described 

 as negative by various physicians and surgeons are to be regarded as 

 failures due to the use of improper culture media, rather than as 

 proof of absence of organisms in the blood or other fluid tested. Why 

 not failures of this kind also in other fields of animal pathology ? In 

 recent years but few serious attempts appear to have been made to 

 isolate a parasite from malignant animal tumors. 



The variety of difficulties encountered in obtaining cultures of the 

 organisms causing tuberculosis, lepra, syphilis, rabies, etc., should 

 also be considered; e. g., pathologists have been satisfied for a long 

 time as to the cause of le])rosy, being able to stain a certain acid-fast 

 organism within the cells, but not until very recently has it been 

 possible to grow it in pure culture and with subcultures therefrom 

 reproduce the disease in mice (Duval: Jour. Exp. Med., 1910, Vol. 

 XII, pp. 649 to 665). 



(4) Failure to demonstrate the supposed parasite in stained sec- 

 tions might be due either to its scarcity, to its indift'erence to stains, 

 to its lack of power to retain them during the washing, or to the fact 

 that it may occur in the tumor in some very minute or unusual form, 



213 



