PAUL F. CLARK 45 



through maturity to old age. These three phases show marked differences in meta- 

 boHc and reproductive rate, and correlated changes in size and structure. Sherman 

 and Albus' have shown that other physiological correlations exist, in that young bac- 

 teria are destroyed by brief exposures to cold and 2 per cent sodium chloride while 

 cells from older cultures are not injured. Differences in agglutinability have been 

 known for many years. There is no evidence that conjugation occurs, but there is 

 ample evidence that, as Child^ has shown in higher forms, youth and rejuvenescence 

 may occur apart from sexual processes. 



Observations such as the foregoing suggest an interesting correlation between the 

 growth of bacterial cells and that of other living organisms. Protoplasmic changes, 

 similar in kind to those cellular changes we associate with youth, maturity, and senil- 

 ity in metazoan forms, occur even in these supposedly "immortal" single cells. Def- 

 inite changes in gross morphology and in finer structure are apparent. A logical as- 

 sumption that bacteria, in common with their more highly organized relatives, regain 

 their youth and vigor of reproduction by the injury and stimulus of conjugation is 

 not borne out by these observations. It must be borne in mind, however, that these 

 observations were all made under conditions admirably suited for growth. Just as 

 numbers of species of fungi have for years been classified with the Fungi imperfecti 

 but later, by more complete studies under varying conditions of growth, have been 

 shown to have definite sexual reproduction and, consequently, have been removed 

 from this family, so it would seem to be highly probable that more detailed study will 

 remove bacteria from their unique position and link them more closely with the rest 

 of the biological world. 



'Sherman, J. M., and Albus, W. R.: J. Bad., 8, 127. 1923; 9, 303. 1924. 

 => Child, C. M.: Senescence and Rejuvenescence. 1915. 



