THE SEQUENCE OF EVENTS 



The Sequence of Events 



These experiments show something more than nuclcar-cyto- 

 plasmic reaction. They show the cytoplasm as the field of reaction of 

 products of different genes accumulated at different rates and over 

 different periods. We may therefore ask, at this point, how it is that 

 nuclear products accumulate in the cytoplasm over long periods. We 

 have already seen that certain particles or determinants, the 

 plasmagenes, can propagate themselves in the cytoplasm with a 

 permanency and autonomy scarcely less than that of the nuclear 

 genes. Wright has argued that growth could not proceed auto- 

 catalytically, nor could differentiated tissues retain their uniform 

 character, without some similar propagation of other cytoplasmic 

 particles. 



Such particles would not necessarily continue permanently and 

 independently. Indeed we have evidence of gradations between a 

 high rate of propagation which leads to permanence and a low rate 

 which leads to transience. These gradations are themselves under 

 genetic control. Thus, as we have already seen. Freer found that 

 in one variety of Paramecium the rate could be forced up to the 

 point at which the kappa particles were unable to keep pace 

 and were gradually lost. In another, the kappa particles reproduced 

 so rapidly that they were never lost, even though the cells 

 of this variety divided nearly twice as rapidly as those of the 

 first one. 



These differences in rates of propagation of cytoplasmic particles 

 — some autonomous and some presumably nuclear products — show 

 how the cell may, at one time, maintain itself and, at another, vary 

 in constitution in the course of development, in the way that we 

 observe it to do. The propagation of any one part of the cytoplasm 

 is thus conditioned by the whole; and, since the whole is modified 

 by the activity of the nucleus, the nucleus is in effect modifying its 

 own activity (Fig. 52). It does so after a delay which must depend 

 on such physico-chemical conditions as the permeability of cell and 

 nuclear membranes, the rates of diffusion of particles of different 

 sizes, and the rates of propagation of nucleo-proteins of different 

 shapes. These are the fundamental conditions of differentiation, and 

 their essential consequence is the lag berv\'een determination and 



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