12 THE PHYSICAL BASIS OF LIFE 



rate and pass to opposite poles (Fig. 4) ; and from the two 

 groups formed, are built up two daugliter-nuclei, while 

 the cell-body divides between them (Fig. 5). Such a proc- 

 ess seems in some respects to contradict all physical j^rin- 

 ciples ; but its meaning has now become perfectly plain. 

 In a general way it means, as Eoux pointed out forty 

 years ago, that the nucleus is not composed of a single 

 homogeneous substance, but is made up of different and 

 self -perpetuating components; and it means that these 

 components are strung out in linear aligmnent in the 

 threads so that they may be divided, or distributed in par- 

 ticular manner, by doubling of the thread.' 



This remarkable conclusion led the w^av in a series of 

 investigations that have brought forth some of the most 

 notable discoveries of our time. Roux's theoretical con- 

 clusion was preceded and perhaps suggested by earlier cy- 

 tological observations which indicated that the nuclear 

 threads contain smaller bodies suspended in a clear basis 

 and aligned in a single, regular series. The further conclu- 

 sion was thus suggested that the longitudinal splitting of 

 the thread might be due to fission of these bodies. Later 

 cytological observations confirmed and considerably ex- 

 tended these conclusions, showing that in some cases the 

 smaller bodies or " chromomeres " aligned within the nu- 

 clear threads are of different sizes and show a constant 

 serial order.' Thus far, cytologists have been able to dem- 

 onstrate these facts in only a rather fragmentary and 

 general fashion (Fig. 6) ; and it seems certain that the 

 visible structure of the threads (only to be seen in sec- 

 tions of fixed or coagulated material) is no more than a 

 rough expression of a finer structure lying beyond the 

 present reach of the microscope. Here, however, genetic 

 research has come to the rescue, bringing forward in- 

 direct proof on a grand scale derived from experiments 



