PAEASITISM AND TISSUE OVERGROWTH 283 



proliferate by substances which destroy or arrest the activity 

 of the higher cells in the same neighbourhood. Thus it is that 

 we can trace various stages of bacterial action from those in which 

 destruction predominates to those in which, as in the infective 

 granulomata, the higher tissue elements undergo destruction, but 

 endothelial and connective tissue elements show, it may be in 

 the very earliest stages of the process, well-marked proliferation. 



Further than this it would seem that we cannot advance 

 with the bacteria. In other words, so far as our present know- 

 ledge permits us to conclude, bacteria — scbizomycetes — below 

 a certain degree of toxicity of their products cannot maintain 

 an existence in the tissues of animals. They succumb to the 

 antibacterial mechanisms of the organism, and, succumbing, 

 any proliferative activity they may have induced comes to an 

 end. In plants, it is true, we meet with definite bacteria living 

 a symbiotic existence in the rootlets, and causing well-marked 

 overgrowths. 1 It may eventually be found that bacteria are 

 capable of initiating progressive cell growth in the higher animals, 

 but of this power proof is so far wanting. 



There are, however, other forms of life which, while of pecul- 

 iarly low toxicity, are capable of existing and actively proliferat- 

 ing within the tissues of animals. Coccidia, for example, are 

 peculiarly common in the rabbit, but the irritation they set up 

 in the tissues is of so mild a type that in many regions the majority 

 of rabbits, young and old, show the results of their presence in 

 the tissues, and that without any sign of disturbance of general 

 health. I need scarcely say that the evidence of their action 

 is especially found in the liver, where, growing within the cells 

 lining the bile ducts, they lead to a proliferation of the same 

 with surrounding overgrowth of the connective tissue, and so 

 give rise to what are truly " cystadenomata." 2 Similar growths 

 of like causation have in rare instances been found in the human 

 liver. It is to be noted that the growths so produced are benign 

 and not malignant. For some reason — what reason remains 

 confessedly problematical — the irritation and the reaction to 



1 [Since this paper was written Dr. Erwin F. Smith of the Bureau of Agricul- 

 ture at Washington has demonstrated conclusively (1907-1916) that a specifio 

 bacterial form, the Bact. tumefaciens, is the causative agent of large localized 

 overgrowths in plants, crown galls, of beets, Paris daisy, roses, etc.] 



* [An adenoma is a tumour reproducing the structure and cell arrangement 

 of one or other type of glandular organ.] 



