ANALYSIS OF FORCES 481 



phase pairing (D., 1929 b). We can now assume that with the same 

 general character in all organisms each type of division works in 

 the same way, on the same mechanical principles, in all organisms. 



Our next task is to relate the two types of nuclear division. 

 Classical cytology was unable to deal with this problem. It was 

 baulked by a conclusion that seemed so obvious that it was never 

 questioned : the homologous chromosomes which did not attract 

 one another at mitosis, came together at meiosis by an attraction 

 which consummated the hitherto incomplete sexual process. An 

 insufficient analysis led to a teleological conclusion and the issue 

 was closed. 



The method that has been followed here is to assume every 

 possible likeness between mitosis and meiosis and then by a process 

 of exclusion to arrive at the essential difference. We have seen that 

 in the attractions and repulsions, in the external and internal 

 coiling relationships of the chromosomes, in the individuality and 

 reproduction of the chromomeres and centromeres, and in the 

 cyclical changes of the spindle and of the nucleus, there is indeed 

 no essential difference between the two types of division. We 

 have seen that on the other hand there is a difference in the timing 

 relationship of the cyclical changes in the nucleus, spindle, centro- 

 meres and chromosomes. This difference can be expressed by 

 saying that in meiosis the changes outside the chromosomes are 

 advanced in relation to the changes inside the chromosomes. In 

 meiosis, as compared with mitosis, the external changes are pre- 

 cocious. On this basis we can assume that the forces acting in the 

 two types of division are the same, and the whole array of observa- 

 tions of mitosis and meiosis becomes available for the comparative 

 analysis of chromosome movement. 



Our third task is to separate this great mass of data into classes 

 which are convenient for handling. Of course, every movement of 

 a chromosome is related in some way with every other, but we have 

 to separate them for individual consideration so far as possible, 

 just as we have to separate the mechanical and physiological 

 activities of the chromosome although these activities continually 

 affect one another. The most important cleavage comes between 

 the external and internal changes in the position of chromosomes. 



R.A. CYTOLOGY. 16 



