yet paired. It is difficult to determine whether this mechanism would fit 

 the Huskins and Newcombe data (1941) and by no means certain that 

 any tension would result from dijfferential contraction. 



Evidence has been put forth which is supposed to show that chiasma 

 formation and genetical crossing over are independent phenomena even 

 in time. This conclusion is based primarily on genetical studies on re- 

 combinations in microorganisms, which seem more easily explained on 

 a copy-choice hypothesis. If this were so, one could consider the problem 

 of chiasma formation without the concomitant problem of crossing over. 

 While this might simplify the problem, the correlations between chias- 

 mata and crossing over seem too good to be fortuitous and we would be 

 left with lack of a reasonable functional relationship for a readily ob- 

 servable condition. On the whole it would seem that knowledge of the 

 forces involved in pairing is a necessary prerequisite to any analysis of 

 factors involved in chiasma formation. 



ENERGETICS OF CELL DIVISION 



Although there is an extensive literature dealing with the physiological 

 aspects of cell division, the metabolic events associated specifically with 

 the initiation of this division and production of energy during the mitotic 

 process are still largely unknown. Critical evaluation of the information 

 concerning this problem can be found in the reviews of Brachet (1957), 

 Bullough (1952), Stern (1956), and Swann (1957). 



The major difficulty one encounters in attempting to analyze the 

 energetics of the cell-division process is that much of the available in- 

 formation is indirect and somewhat controversial. It is well established 

 that some cells will undergo mitosis in the absence of oxygen or in the 

 presence of respiratory inhibitors (i.e., carbon monoxide, azide, cya- 

 nide), while others will not (Swann, 1957). Mitosis is not initiated in 

 sea urchin eggs under anaerobic conditions or in the presence of carbon 

 monoxide. On the other hand, egg cells containing large amounts of 

 yolk such as those of the frog and trout, undergo extensive cleavage in 

 the complete absence of oxygen. In the case of the frog egg, cleavage 

 will continue even when 90 per cent of the respiration is inhibited by 

 carbon monoxide. The effect of oxygen lack on mitosis in tissue cultures 

 is no clearer. Some workers have reported rapid inhibition of mitosis in 

 tissue culture cells deprived of oxygen, while others have observed cell 

 division to occur for a time in the absence of oxygen. The effect of 

 oxygen deficiency and metabolic inhibitors on active mitosis, while 

 somewhat more consistent, is almost as difficult to interpret. In the case 



MECHANICS AND PHYSiOLOGY OF CELL DIVISION / 155 



