REPULSION EQUILIBRIUM 503 



first case the threads have run parallel, in the second they have 

 made a complete revolution round one another. In both cases 

 they are distinct. A single continuous dicentric ring formed by 

 their making half a revolution does not apparently arise. We are 

 driven to conclude that there is an absolute distinction between a 

 parent and a daughter-thread in the reproduction of the chromo- 

 some. 



(ii) Simple Repulsions in the Nucleus, {a) Between Chromosomes 

 and Configurations. During the prophase of mitosis and meiosis 

 the chromosomes are limited in their movements by the nucleo- 

 cytoplasmic surface. When the enclosed space is small, as it 

 usually is, the separate chromosomes or configurations arrange 

 themselves equally so far as their internal movements and occa- 

 sional interlocking permit. Such an evenness of distribution at 

 mitosis led Lillie (1903, cf. 1905) to suppose that they bore a surface 

 charge and therefore repelled one another {cf. Hardy, 1911). This 

 conclusion is supported at once by the arrangements found in the 

 large prophase nuclei of egg-cells and embryo-sac mother-cells. 

 Here an even distribution is no longer attained. The chromosomes 

 are further apart than in corresponding small nuclei, but they are 

 by no means equally distributed {cf. Gustafsson, 1935). Evidently 

 the distances are too great for the repulsions to be effective. More 

 critical evidence of the effect of surface charge, however, is provided 

 by the movements occurring within configurations during the 

 prophase of meiosis. 



{b) Within Configurations : Terminalisation. The configurations 

 of bivalent chromosomes found at diplotene present us with the 

 largest field for accurate comparison of quantitative data in cytology. 

 The configurations make up an isolated and protected system 

 within the nuclear membrane. They vary in numbers of chiasmata 

 that can be counted, in sizes of chromosomes that can be measured ; 

 they occur in nuclei of all sizes, in both sexes and in all organisms 

 from Amoeba to man. The observation of systems at this stage 

 therefore constitutes a natural experiment in cell physiology capable 

 of establishing the principles governing the external mechanics of 

 the chromosomes under the simplest and most readily comparable 

 conditions. 



