FACTS AND PROBLEMS OF IMMUNITY 309 



severely and detrimentally infecting the animal body. In view of this 

 unquestioned fact, the teaching which considers all poisonings as due 

 either to true soluble secreted poisons, or to true endotoxins liberated 

 only on disintegration of the bacterial cell, is probably too narrow; and it 

 would seem not unlikely that many organisms, possibly all, secrete bodies 

 which are not soluble in their condition at secretion in culture media or 

 in the body fluids, but which are susceptible to digestion in the animal 

 body, and may thus become soluble and assimilable, and when toxic act 

 harmfully on the body cells. This question is an important one and 

 will be considered later. Besides these actively poisonous bodies which 

 we have been considering, there are probably bodies such as some at 

 least of the substances called aggressins by Bail, which, while not being 

 toxic in themselves for the animal body, nevertheless are active defen- 

 sive agents of the bacteria, probably neutralizing certain bodies of the 

 animal economy, which are indirectly injurious to the bacteria. Further 

 than this, certain bacteria may be furnished with envelopes, capable 

 possibly of protecting them either chemically or physically from harm- 

 ful influences. 



Some clarity of conception may, as we have suggested, be gained 

 by comparing some of the products of pathogenic bacteria with bacterial 

 pigments and with insoluble interstitial or intercellular substance, 

 which may be seen accompanying bacteria in cover-glass prepara- 

 tions. Soluble toxic secretions are to be compared to such pigments 

 as the pyocyanin of Bacillus pyocyaneus, which is so readily soluble in 

 culture media; endotoxins proper, to pigments confined to the bac- 

 terial cell or, at least when secreted, being insoluble in culture media, 

 such, for instance, as the weU-known red pigment of Bacillus prodigiosuSj 

 which may often be seen free among the bacteria in irregular red gran- 

 ules like carmine powder. That bodies such as this latter might be 

 extruded from pathogenic bacteria, and not be soluble in the usual cul- 

 ture fluids, is not improbable, and the fact that more or less insoluble 

 interstitial substances are not infrequent among bacteria is well known. 

 Among pathogenic germs these characters are often more marked in 

 freshly isolated cultures. The sticky, almost shmy character of cul- 

 tures of meningococcus may be recalled, a character which tends to 

 disappear after a few generations of artificial cultivation, and the highly 

 mucinous capsule of the Streptococcus mucosus which tends to decrease 

 under artificial cultivation, as do also the capsules of pneumococci and 

 streptococci. 



Now, it seems — and this view has been supported by Walker, Deutsch, 



