H. G. Thornton 249 



Before investigating the action of varied external conditions on the 

 "spreading," the process by which the organism produces the normal 

 spreading colony on the surface of meat-extract peptone agar was care- 

 fully studied. The germination of spores and the early stages in the divi- 

 sion and grouping of the cells were observed by means of the agar block 

 technique described by Hill(iO). 



In order to observe the formation and subsequent spreading of a 

 colony, platings of sterile nutrient agar were poured and were inoculated 

 at the surface with spores of B. dendroides. The plates were incubated at 

 37° C, and were examined at short intervals under a |-inch objective. 

 In this way the development of a surface colony could be observed on 

 the actual plating. Owing to the extreme rapidity with which B. den- 

 droides grows, no difficulty was experienced from air contaminations, 

 which had no time to develop during the short period concerned. 



When incubated at 37° C. on the surface of nutrient agar, the spores 

 germinate in about 45 minutes, producing rods which are at first non- 

 motile. The rods divide rapidly and the daughter cells do not form chains 

 but come to lie side by side so as to form packets of four to six cells. 

 A young colony at this stage has mosaic-Hke appearance under a low 

 power, owing to the packets of rods lying at divergent angles. The forma- 

 tion of these packets is not uncommon in other organisms and is described 

 and illustrated by a number of authors. Their production is probably 

 conditioned by surface tension and has an important influence on the 

 development of the colony. 



At about this time a water film, covering the colony, becomes notice- 

 able. It seems probable that this is derived from the saturated atmo- 

 sphere covering the agar film, the young colony acting as a point upon 

 which condensation occurs. The surface growth of the organism possesses 

 the power of retarding the absorption of this water by the underlying 

 agar. The following experiment illustrates this. Droplets of distilled 

 water were placed on a portion of the agar over which a colony was 

 spreading and other drops of similar volume upon a sterile portion of 

 the plating. The former took four to six times as long to disappear as 

 the latter. The time taken for water to be absorbed was found to vary 

 according to the thickness of the agar film and other conditions but was 

 always several times greater within the area of a branching colony than 

 outside it. The absorption of water within the colony area was found to 

 be retarded not only where the surface was entirely covered with bac- 

 terial growth but also in places where a large fraction of the surface 

 consisted of apparently uncovered agar lying between branches of the 



