CH. IV] VARIATIONS IN MORPHOLOGY 39 



influence is removed, or in response to a particular stimulus — 

 such as the attraction of a boxing-match — may assume the 

 form of an irregular crowd concentrically arranged. A certain 

 controlling influence in the case of the miners, or a common 

 spontaneous impulse, may result in their marching in military 

 formation. In other words, a collection either of the pathogenic 

 organisms or of the harmless pigment producers, may assume 

 temporarily a formation rightly regarded as characteristic of 

 the other; but we should be mistaken in supposing on this 

 account that the soldiers were being transformed into miners, 

 or vice versa. 



The development of zoogleic forms may occur spontane- 

 ovMy, or it may be brought about artifidaUy. 



I. Zoogleic Forms Occurring Spontaneously. 



These may represent a regular phase in the life history 

 of the organism; on the other hand, they may occur quite 

 irregularly as an occasional variation — either in cultures on 

 artificial media or in the living tissues — in which case one must 

 regard the change as representing a phase in the life history 

 of the organism at an earlier stage in its evolution. 



Perhaps the earliest account of zoogleic forms occurring 

 in the life history of a micro-organism was that given by 

 Ray Lankester in 1873, with reference to the non-pathogenic 

 Bacterium rubescens. The units of this bacterium were 

 observed to become aggregated into a multitude of forms, 

 protean in their variety — stellar, globose, massive, arborescent, 

 catenular (or chain-like), reticular, tessellate and so on. (Dia- 

 grams of each of these forms are appended to the original 

 article.) 



The tvberde bacillus indicates its relationship to the 

 streptothrices by forming in old cultures a branching filament, 

 sometimes with "clubbed" ends, while in the living tissues, 

 under certain conditions, it gives rise to a radiating structure 

 similar to that of the actinomyces (Muir and Ritchie). 



The bacillus of glanders, similarly, on artificial culture may 

 exhibit short filamentous forms, and under certain conditions 



