PAUL F. CLARK 41 



typhoid group and V. cholerae. He found that this is a common occurrence in the 

 stage of rejuvenation of cultures of pathogenic bacteria, but found no evidence that 

 these Y-forms are part of a complex life-cycle. 



The fact that so many of the common pathogens have, in recent years, been passed 

 through infusorial earth and porcelain filters especially after anaerobic growth or 

 through the influence of bacteriophage is additional evidence in favor of the occur- 

 rence of minute forms which under suitable conditions are capable of growing into 

 typical organisms. Apparently some nuclear reorganization does occur, but that con- 

 jugation is an essential precursor does not seem to have been proved. 



Filterability is admittedly not the best basis for judgment as to the size of particles, 

 since electro-physical phenomena play such a large part in the results (Kramer,' 

 Mudd)-. But when filtrations are carried out under more or less standardized condi- 

 tions with silica filters and bacteria suspended in physiological saline, the positive 

 results obtained by a number of observers certainly tend to indicate the occurrence 

 among our common pathogens of filterable bodies smaller than the type forms com- 

 monly recognized. 



Uninterrupted study of the developing organisms by some hanging-block method 

 (Hill,^ OrskoVji Hort,^ Gardner'^) or by moving pictures (Bayne- Jones, Bronfenbren- 

 ner)7 will continue to be essential to the growth of knowledge of the morphology and 

 development of bacteria. Li such studies, however, individual cells quickly become 

 lost in the "log jam" of the dividing bacteria so that clear observation is difficult. An- 

 other method which has given some information in regard to the morphological 

 changes during the growth of bacteria is the examination of smears removed at fre- 

 quent intervals from cultures growing under uniform conditions, the measurement of 

 large numbers of the individual bacteria, and the reconstruction of the life-story large- 

 ly by statistical methods. The common notion of the "normal" morphology of bac- 

 teria is based upon the inspection of material from original habitats or from cultures 

 which have been growing approximately twenty-four hours or some multiple of that. 

 The writer has pointed out the general failure to appreciate this time factor and the 

 resulting errors in our conception of the morphology of the common bacteria. Clark 

 and RuehP have shown that even on the ordinary standardized media bacteria pass 

 through striking morphological changes which are coincident with the different growth 

 phases described by Lane-Claypon' in the life-history of a bacterial culture. Later, 

 Henrici"* confirmed these findings and worked out many points more completely. 



'Kramer, S. P.: /. General Physiol., g, 811. 1926. 



= Mudd, S.: J. Bad., 8, 459. 1923. 



3 Hill, H. W.: J. Med. Research, 7, 202. 1902. 



''Orskov, J.: /. BacL, 7, 537. 1922. 



s Hort, E. C: see various works cited previously. * Gardner, A. D.: loc cit. 



7 Bayne- Jones, S., and Tuttle, C: /. Bad., 14, 157. 1927; Bronfenbrenner, J.: Meeting Soc. 

 Path. 6* Bad. April, 1927. 



« Clark, P. F., and Ruehl, W. H.: J. Bad., 4, 615. 1919. 



9 Lane-Cla>TDon, J. E.: J. Hyg., 9, 239. 1909. 



"Henrici, A. T.: Proc. Soc. Exper. Biol. &= Med., 19, 132. 1921; 20, 179, 293. 1922-23; 21, 215, 

 343, 345- 1923-24; 22, 197. 1924; Science, 61, 644. 1925; /. Infed. Dis., 37, 75. 1925. 



