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- 

 ously, or it may be brought about artificially. 



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, 

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

 grams of each of these forms are appended to the original 

 article.) 



The tubercle 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 



