104 S. S. COHEN 



As noted earlier, a growth pattern of this type suggests an integrated 

 interdependence of polymer syntheses in cell development, since concentra- 

 tion of the investigator upon the substance of the entire culture instead of 

 upon individual cells tends to obscure the discontinuities of growth and 

 division. Such an appearance of apparent integration and interdependence 

 of the parts has even been considered to be a general law, as in the analysis 

 of the induced biosynthesis of enzymes in bacteria which produce the new 

 protein under conditions of gratuity. Thus, in a culture containing bacteria 

 randomly distributed in various phases of growth and division and growing 

 in a substrate which neither induces nor otherwise affects the enzyme under 

 consideration, enzyme production is proportional to the increase of mass of 

 the culture. One might ask if such a result would be obtained in all phases of 

 a synchronized culture. It will be noted below that this result is not obtained for 

 the hght-stimulated pigment synthesis that occurs in photosynthetic bacteria. 



A similar conclusion concerning the integration of polymer synthesizing 

 mechanisms is often drawn from the observation that bacteria in exponential 

 growth may tend to excrete few of their metabolites, i.e., there is little waste, 

 since the enzymes which govern later reactions are produced and operate in 

 such a way as to handle earher metabolites most efficiently. This type of 

 result only suggests that the system mider mvestigation is selected auto- 

 matically for just these properties and prunes off the more wasteful and less 

 efficient members of the culture by growing and multiplying at a maxima] rate. 



This apparent balance of cell syntheses during exponential growth may 

 often be changed by changing the nutritional conditions. For example, 

 growth of E. coli in lactate instead of glucose wiJl produce curves of expon- 

 ential growth of cell number and cell mass which are not parallel. The 

 bacteria are found to be decreasing in size at each division. With such cultures 

 a lag in cell division, which occurs before the period of exponential increment 

 begins, represents a period of cell enlargement to an apparently critical size 

 before division can start. 



In vegetative growth in diatoms, imequal division is a common pheno- 

 menon. The cell wall consists of two rigid half-waUs, which in most of these 

 forms fits one within the other. At division, the halves separate and a new 

 half-wall is secreted within each. Thus, a smaller half-cell must be produced 

 at each division and cell size becomes progressively smaller. A stage is 

 reached in which a relatively labile large protoplast is produced, which leaves 

 both parts of the cell wall behind and eventually deposits new, large half-cell 

 walls around itself. 



B. Differentiated Cells 



As remarked earher, it is difficult to handle differentiated cells for purposes 

 of virology, although the possibihties of effecting an experimental separation 



