822 



SCIENCE. 



[N. S. Vol. XX. No. 520. 



termination of the disease there are no 

 new bodies present, but the antibodies are 

 on hand in relative abundance. The situa- 

 tion which the invader has to face is thus 

 qualitatively the same at the beginning 

 and at the end of the attack. How does 

 he meet it by defensive methods^ 



Three possible fates await the invaders: 

 (1) They are largely destroyed within the 

 body; (2) they are excreted, or discharged 

 through various channels; (3) they remain 

 indefinitely in the body after the disease 

 is over, to be eventually destroyed or elimi- 

 nated. 



That the microorganisms are largely de- 

 stroyed within the body in the course of 

 the disease is not open to dispute; this 

 class is of no special significance to us. Of 

 most importance are those that escape to 

 continue their life cycle in another sub- 

 ject. The mechanism of elimination is of 

 vital importance to the parasite. It as- 

 sumes many forms, and is admirably 

 adapted in the various specific diseases to 

 perpetuate the existence of the species. 



The survival of the microbes after the 

 disease is over may be explained partly on 

 the ground that in nearly all diseases some 

 of the microbes pass their final stage near 

 the surface of the skin, or mucous mem- 

 brane, or in organs in direct or in indirect 

 contact with the outer air, so that escape 

 outward is readily effected through de- 

 struction of tissue, and hence protection 

 from the bactericidal forces of living tis- 

 sue. The small number which in some 

 types of disease remain alive for some 

 time after the disease process has subsided, 

 may also be enclosed in small foci of 

 necrotic tissue, and thus escape destruc- 

 tion temporarily. 



I am inclined to believe, however, that 

 among the problems of the future will be 

 the elucidation of still another mechanism 

 for the protection and escape of the micro- 

 organism. It is highly probable that in a 



certain number of species of bacteria after 

 the active vegetative stage a latent stage 

 follows, during which the parasite which 

 has escaped destruction provides itself 

 with some protective envelope which also 

 aids it in its passage to a new host. This 

 envelope, which may be some specific sub- 

 stance not recognizable with the micro- 

 scope, or which may be represented by the 

 capsules in some groups, may be a defen- 

 sive body of the parasite stimulated to over- 

 production by the antibodies of the host. 

 This body also interferes with the metab- 

 olism of the microbe and thus acts in the 

 double capacity of stopping the disease and 

 protecting the microbe at the same time. 

 This hypothesis suggested itself to me while 

 endeavoring to account for the peculiar 

 behavior of tubercle bacilli under culti- 

 vation. 



It is well known that tubercle bacilli 

 from the diseased tissues of cattle grow 

 very slowly, and then only upon certain 

 culture mediums, such as blood-serum. 

 After several years of continuous cultiva- 

 tion they multiply vigorously in glycerin 

 bouillon and can hardly be distinguished 

 in appearance from those human varieties 

 of the bacillus which grow richly from the 

 first or second transfer. There seemed to 

 be no justification to assume that the bacil- 

 lus had completely changed its metabolism 

 under artificial cultivation. The more 

 rational hypothesis seemed to be the one 

 which assumed the existence of some pro- 

 tective substance only slightly acted upon 

 by the serum, not at all in glycerin bouil- 

 lon, and therefore a hindrance to multi- 

 plication. After repeated transfers, this 

 protective substance was slowly lost either 

 through a selection of bacilli or absence of 

 stimulation on the part of the host, or both 

 causes combined, and the growth became as 

 luxuriant as with the more saprophytic 

 human varieties. It is obvious that each 

 group or species of bacteria would have its 



