THE PLAN OF CELLULAR REPRODUCTION 171 



discussed, certain specific preparations for division are required. One 

 of these is the provision of proteins involved directly in the division 

 process— the proteins of the mitotic apparatus. A glance at a dividing 

 cell shows that the working elements of the mitotic process— the 

 spindle, etc.— are rather large, and our studies on the chemistry of the 

 mitotic apparatus show that it is composed of proteins with which RNA 

 is associated and which amount to a large portion ( 10 per cent in the 

 case of the sea-urchin egg) of all the proteins of the dividing cells. Dr. 

 Hans Went has shown that these proteins may be characterized im- 

 munologically, and that they are present in the sea-urchin egg before 

 the visible onset of division. Dr. Ellen Dirksen has investigated the 

 possibility that the RNA component of the mitotic apparatus was syn- 

 thesized in the course of division, and has obtained negative results. 

 E. W. Taylor (1959) has shown that chloramphenicol will inhibit the 

 formation of the mitotic spindle in newt fibroblasts if applied some 

 time before the spindle actually begins to form, but the drug is inef- 

 fective at the time when the spindle is forming. In short, we have rea- 

 sons to think that the preparation of the molecules which will be as- 

 sembled into a mitotic figure takes place- some time before mitosis and 

 is a prerequisite to division. 



A second rather definite preparation for division, in some cells at 

 least, is the provision of an "energy reservoir," as Swann ( 1958 ) calls it, 

 for the division process. It is a remarkable fact, in many cases, that in- 

 hibitors of oxidations and phosphorylations have little or no effect on 

 division once it has begun but can prevent it if applied before a certain 

 "point of no return," which occurs some time before division. A great 

 deal of the experimental work on this question has been done by Dr. 

 Zeuthen, and perhaps he will discuss it in more detail in this sym- 

 posium. The general idea that the energetic price of division has to be 

 paid in advance has a great deal of evidence behind it, and it must ap- 

 pear on any timetable and balance sheet of cell reproduction. 



A good many other specific preparations for division could be 

 hidden away in the interphase period, mixed up with those growth 

 processes that are not specifically related to division. For example, we 

 know nothing about the pre-conditions of the changes in the nucleus, 

 such as the coiling of the chromosomes, which we can detect only when 

 they reach a rather obvious stage but which must be set in motion much 

 earlier. 



To summarize the emerging view of the plan of the reproductive 

 cycle of the cell, I have constructed a time map ( Figure 4 ) which can- 

 not be very precise when applied to all cells but might be very precise 

 if it were drafted for a single kind of cell. The important point is that 

 the "stimulus" to the actual cell division process need be nothing more 

 than the completion of the last prerequisite preparation for division. 



