18 A. C. R. Dean and Sir Cyril Hinshelwood 



cells. We have set forth elsewhere evidence in support of our 

 view that this is an oversimplified, and sometimes incorrect 

 interpretation of the phenomenon (Dean and Hinshelwood, 

 1957). Our present views, and the arguments for them, may 

 be summarized as follows. 



(1) Secondary colony formation is essentially a pheno- 

 menon shown by the ageing primary colony. In this we are 

 in complete agreement with the work and views of Haddow 

 (1937). 



(2) Observations on the very early stages of colony 

 growth show that cells align themselves into more or less 

 regular arrays giving the colony a characteristic internal 

 structure. This is generally very close-packed. 



(3) Growth of the colony stops when nutrient is exhausted 

 or toxic products accumulate. Renewed growth is only pos- 

 sible when one of several things has occurred. The cells have 

 thrown off mutants or have adapted themselves to utilize 

 a substrate not utilized at first, or they have become resistant 

 to something which has hitherto impeded their growth, or 

 regions of lysis occur permitting cannibalism in parts of the 

 existing colony, or cracks and channels may develop in 

 the mass of the colony allowing nutrient to diffuse from the 

 medium below to its surface so that fresh growth can take 

 place there. 



(4) The renewed growth can result in papilla formation, 

 when for any of a number of reasons it is localized. If, for 

 example, capillary channels to feed nutrient to the aerated 

 surface of the colony are required, the papillae will occur at 

 the points where such "craters" exist. The surface may be 

 preferred to the periphery because the concentration of toxic 

 products is lower. Particular points on the periphery may be 

 preferred for such reasons as that the highly heterogeneous 

 microstructure of the agar gel there provides adsorption sites 

 which remove inhibitors, or concentrate growth factors. 



(5) If renewed growth depends in this way on fortuitous 

 structural factors, the new array of cells will not conform 

 to the former one in orientation and packing, and a visibly 



