168 CELLS, TISSUES, AND ORGANISMS 



of the centers, for we shall have to fit this into our timetable of the cell 

 cycle. The details of the experiments have been published (Mazia, 

 Harris, and Bibring, 1960), and I need only consider the essential ex- 

 perimental designs and the conclusions. In the quadripartion experi- 

 ments on sea-urchin and sand-dollar eggs, we interpreted the four-way 

 mitosis as indicating that four potential centers were already in exist- 

 ence at metaphase. We also interpreted the events following four-way 

 division as suggesting that mercaptoethanol blocked the conception of 

 new potential centers. If these interpretations are correct, then we 

 should be able to find a time before metaphase at which blockage 

 would be followed not by four-way division but by two-way division— 

 a time when the new units have not yet been conceived. With this ex- 

 perimental design we were able to show that the conception t)f new 

 centers takes place at about the time of anaphase or telophase. That is, 

 the conception of new centers for the next division is taking place dur- 

 ing later stages of the previous division. In fact, this is the earliest 

 event we can assign to a given division. 



We can also, using the same experimental system, establish the 

 time when the daughter centers become capable of splitting from their 

 parents and forming poles. Essentially, the experiment is given by 

 Figure 2, where we see that after a certain time following metaphase, 

 the double units at the poles of the blocked cell begin to split apart. 

 Interestingly enough, the time is about the same as the time of concep- 

 tion of the new centers, and this conclusion is included in the general 

 reproductive scheme shown in Figure 3A. Normally, new centers are 

 conceived at about the time of parturition of the old, but we cannot say 

 that the two processes are connected causally. Clearly, parturition can 

 take place without simultaneous conception, but we do not know 

 whether the reverse is true. 



Finally, Dr. Bucher and I ( Bucher and Mazia, 1960 ) investigated 

 the possibility that the reproduction of the centers, which precedes the 

 reproduction of DNA, as we shall see, was involved in the control of 

 the latter. The answer was negative. When we blocked the conception 

 of new centers with mercaptoethanol, this had no efiFect on DNA syn- 

 thesis. Thus the two main reproductive events of the cell cycle do not 

 seem to be linked causally, even though they must be coordinated in 

 the normal course of events. 



The study of the reproduction of the centers is of particular interest 

 to the student of cell reproduction for a number of reasons. Obviously 

 it is an absolute prerequisite to division, in animal cells at least, and 

 therefore we must know its place on the time map of the cell cycle. But 

 it is also a model for the reproduction of cytoplasmic particles, and it 

 may be more than just a model if all of the important self-reproducing 

 particles of the cytoplasm are related, as they may well be. 



