142 CELLS, TISSUES, AND ORGANISMS 



medium is. After nine shocks the average cell size may be 1.5 to two on 

 medium A, two to four on medium B and perhaps six to ten on medium 

 C. The average logarithmic cell has size one. This dramatic eflFect on 

 growth is only to a relatively small degree reflected in the results of 

 Figures 4 and 5. 



For synchrony to be induced, the average cell must grow some. 

 But the synchrony is not a simple function of the size increase of the 

 cells during the cycling temperature or of the number of temperature 

 shocks applied per unit time or per unit growth. The results are com- 

 patible with the view, that, depending on the quality of the medium, 

 the average cell performs enough growth during three, five, or seven 

 temperature cycles (Figure 6, C, B, A) to permit at least one syn- 

 chronous division in all, or almost all, cells. The total growth may be 

 taken as a rough measure of the synthesis of a number of molecular— 

 mostly macromolecular— substances essential for division. We can list 

 protein ( Christensson, 1959), both of the nucleic acids (Scherbaum, 

 1957b; Iverson and Giese, 1957 ) , DNA— also nuclear volume, whatever 

 this represents in terms of molecules (Zeuthen and Scherbaum, 1954) 

 —and nucleoside-triphosphates (Plesner, 1956, 1958a, 1958b). Even 

 though there are suggestions to the contrary (with regard to DNA: 

 Scherbaum, Louderbach, and Jahn, 1959), the present author sees no 

 strong evidence ( cf. Hamburger and Zeuthen, 1960, and text for Figure 

 13) that any of these groups of substances limits division. After the 

 cycling heat, the cell carries more than double the normal average 

 amount of them all. Still, the population divides synchronously at con- 

 stant 28° C. only after a lag of about 80 minutes. The possibility re- 

 mains that the syntheses of small fractions or molecular segments are 

 specifically interfered with by the intermittent heat, so that qualities 

 rather than quantities change. In both cases, the cycling heat would 

 bring the system out of balance, so that one or more chemical or physi- 

 cal conditions for division fail to mature. At 28° C, maturation for di- 

 vision occurs in standard time also in cells which before the final shock 

 are transferred to tap water or to a simple inorganic medium. It is in- 

 terfered with in the same way by heat in both media ( Hamburger and 

 Zeuthen, 1957). 



From what has been said, it is clear that the first synchronous di- 

 vision may be, or even is likely to be, special. More about this later. 

 Holz ( 1960) studied mating type I, variety I, and found that the struc- 

 tural events between the first and the second synchronous division fol- 

 low the same time-course as in the logarithmic cell. Also, he pointed 

 out that, except for the absence of a micronucleus in strain GL, the 

 time-course of the structural events is similar in GL and in mating type 

 I, variety I, both non-synchronized. 



Still, in strain GL the simple measurement of times between sue- 



