DISSOCIATION AND TRANSFORMATION 161 



Dawson Classification 



A dissociation form, other than the S and R forms, was de- 

 scribed in 1934 by Dawson. 303 It appeared to be a mucoid variant 

 of Pneumococcus and was strikingly different from the two main, 

 accepted variants. Dawson intimated that the terminology of bac- 

 terial dissociation should be changed to include the M form. 



In later communications (1934*), Dawson 302 " 3 gave many more 

 details of the several stages of pneumococcal dissociation. He 

 showed, first, that the change from the typical, virulent form to 

 the degraded variant was not a simple direct S — » R conversion, 

 but that the dissociative process consisted of several phases. In 

 this cycle there were three outstanding stages represented by dis- 

 tinct difference in colony appearance and morphology, and here he 

 departed from the orthodox concept of the S and R forms. 



At first reading, Dawson's discussion and proposals are a little 

 bewildering. He makes the apparently radical suggestion that the 

 old designations smooth and rough be changed and the term "mu- 

 coid" be introduced into the terminology as it applies to Pneumo- 

 coccus ; thus, S would become M ; R would become S ; and the new 

 form would be R. His revelation of the intricacies of the dissocia- 

 tive phenomena and the proposed change in terminology are apt to 

 cause some confusion in minds accustomed to the accepted order 

 of dissociative nomenclature. But an unprejudiced and painstak- 

 ing study of the facts and his recommendations serves to dispel 

 some of the doubts raised in a cursory reading of the text. Daw- 

 son's contentions were founded on the appearance of a new variant 

 during the cultivation of an R form of Pneumococcus originally 

 derived from a Type IIS culture. When this strain was diluted, 

 thinly streaked on blood-agar plates, and incubated several days 

 at 37°, many of the colonies showed evidence of a variety of sec- 

 ondary growths. The following is taken from Dawson's description 

 of the S -» R transformation. 



For convenience the evolution of the R variant may be described in 

 several stages although it is emphasized that the process is both gradual 



