H. G. Thornton 253 



seen in the living condition and has also been studied in microtome 

 sections of the processes, prepared by the technique developed by 

 Legroux and Magrou (H). It is uncertain whether the outward movement 

 of the cells is due to the repelhng influence of substances produced at 

 the centre of the colony or to attraction by some factor in the medium 

 outside. 



The branching colony thus produced, spreads over the surface at a 

 surprising rate. Thus, on a nutrient agar plate incubated at 37° C. a 

 colony has been observed to increase in diameter from 0-6 cm. to 1*75 cm. 

 within two hours. 



Under conditions favourable to the organism, the spreading growth 

 may continue until the entire surface of the plating is covered with 

 growth. As a rule, however, if the area of a spreading colony be measured 

 at intervals, it is found that a short lag period is followed by rapid 

 spreading expansion which soon becomes increasingly slower, until it 

 finally ceases after a period which varies according to the conditions of 

 growth. 



On nutrient agar, the cessation of spreading is accompanied by a 

 change in the morphology of the organisms. Long chains of cells are 

 produced within which endospores soon appear. The formation of these 

 chains of sporangia produces a marked alteration in the appearance of 

 the colony under a low power. At the tips of the branches, the edge is 

 now composed of a mass of convoluted filaments, similar in appearance 

 to those that compose colonies of Bacillus anthracis. Growth at the 

 extremity of the branch now takes place laterally, so that the tip expands 

 and becomes club-shaped. In the interior of the colony no further 

 growth occurs, the cells forming endospores. 



With the object of discovering a means of checking spreading growth 

 it was important to ascertain the cause which normally brings about the 

 slowing and final cessation of spreading. In the course of many experi- 

 ments, it has been found that the length of time during which spreading 

 continues is not noticeably affected by the quantity or nature of the food 

 constituents of the medium, although, as will be shown, the amount of 

 spread occurring within that period is greatly influenced by these factors. 

 This suggested that the cause was not nutritional but rather of a physical 

 nature. From our knowledge of the method by which the spreading 

 occurs, through actual motility of the cells in the moisture film, it 

 seemed probable that the cessation of spread was due to the evaporation 

 of this surface moisture. That the drying up of this water film is able to 

 arrest the spreading was shown by the following experiment in which 



