THE PLAN OF CELLXH^AR REPRODUCTION 165 



units separated to form a four-pole mitotic figure. When the conditions 

 for mitotic movement of the chromosomes were restored, the four-polar 

 figure functioned to divide the cell into four. 



The first interpretation of these observations was that the centrioles 

 had undergone an extra replication in mercaptoethanol, while the 

 chromosomes remained arrested at metaphase. But further observation 

 showed dramatically that this was not the case. My colleague Thomas 

 Bibring observed the further history of the cells produced by quadri- 

 partition and observed that when they first made a mitotic figure, the 

 figure had only one pole ( Figure 3B ) . Division with one pole is impos- 

 sible, and so this mitotic cycle was abortive; the ceils re-entered inter- 

 phase, presumably completed another cycle of replication of the cen- 

 ters, and now entered mitosis the second time with two poles and 

 divided normally. The interpretation is simple. These four cells, pro- 

 duced by the four-way mitosis, have received only the total number of 

 centriolar units that ordinarily would have been passed on to two cells. 

 Each had half the normal centriolar inheritance, and therefore could 

 only make half a mitotic apparatus! The conclusion, therefore, is that 

 the centers did not duplicate during the mercaptoethanol block. Rather, 

 two units which already existed at each pole split apart and separated. 

 Two poles composed of four potential poles were given a chance to 

 form four actual poles. 



From these observations we deduce: (1) that the doubleness of 

 the centers is not actually required for their mitotic operation; ( 2 ) that 

 the splitting and separation of the paired centers is quite a diflFerent 

 process from the synthetic determination of new centers, the former 

 being insensitive to mercaptoethanol; and (3) that the minimum re- 

 quirement for obtaining the four actual poles from the two pairs of 

 poles is the provision of time. That is, we need not suppose that the 

 mercaptoethanol block served any function other than to arrest the 

 division while the development and splitting of the centers proceeded. 



If these deductions are correct, they lead to the conclusion that the 

 over-all reproduction of the centrioles is a generative kind of reproduc- 

 tion ( Figure 3A ) . We imagine that the first event, an event which in- 

 volves genuine molecular replication and may be designated as the con- 

 ception of a new center, gives rise to a unit which is not a functional 

 center but is capable of developing into one. This event may be in- 

 hibited by mercaptoethanol, but once it has taken place, the further 

 development has diflFerent properties and is no longer so inhibited. The 

 time lapse between the conception of a unit and its parturition (that 

 is, its becoming able to function independently) is that required for 

 the development of the daughter unit; the relation between the prod- 

 ucts of the reproduction is not that of sisters but of mother and daugh- 



