PASSIVE AGGRESSIVITY 37 



better position to maintain themselves and to multiply in the second, 

 and as the transfers are continued through a series of animals, it 

 will be observed that the number of organisms which is necessary 

 to kill the animal becomes progressively smaller, and the period of 

 incubation, i. e., the interval elapsing between infection and the first 

 evidence of the resulting disease, shorter, until finally a strain is 

 obtained in which the degree of virulence can no longer be increased 

 by animal passage this constitutes the virus fixe in the sense of 

 Pasteur. 



Potential Virulence. While we have thus gained a material basis 

 for our concept of the actual virulence of an organism, it is important 

 also to recognize a certain potential virulence, viz., the ability of an 

 organism actually to form capsules when placed under conditions 

 which, cceteris paribus, are favorable to their development. Evi- 

 dently only those organisms of the capsule-forming group, or at any 

 rate those in which an hypertrophy of the ectoplasm can occur, are 

 capable of acquiring a notable degree of virulence in which this 

 potentiality is inherent. If once this is permanently lost the organism 

 in question is manifestly non-virulent, so far as its actual develop- 

 ment in the infected animal is concerned. We have had an excellent 

 illustration of this in Horiuchi's experiment, referred to above. To 

 determine this potentiality it is sometimes only necessary to grow 

 the organism in serum-containing media and to examine micro- 

 scopically for capsules. In the non-capsule formers, on the other 

 hand, microscopic examination is insufficient to determine whether 

 an organism under consideration is virulent or not; in that case the 

 animal experiment alone will decide the question. 



Factors Determining Capsule Formation and its Significance. 

 Of the factors which are operative in determining the formation of 

 capsules very little is as yet known. One could imagine, of course, 

 that as the result of favorable changes in nutrition, certain biological 

 changes would result of which the hypertrophy of the ectoplasm 

 is one of the consequences; in other words, that capsule formation 

 is an index of a condition of particularly active nutrition. There 

 are certain facts, however, which suggest that this explanation is 

 not correct. We find that capsule formation may be evoked by agents 

 which have no nutrient properties whatever. Danysz thus found that 

 the anthrax bacillus when grown in arsenical media of increasing 

 concentration forms enormous mucinous capsules which protect 



