482 CELL MECHANICS 



The first consist in movements of individual chromosomes in relation 

 to each other and to bodies outside them. These movements are 

 the data of external mechanics. The second consist merely of changes 

 in shape of the chromosomes. They are the data of internal 

 mechanics. 



A third class of data arises from the division into external and 

 internal mechanics. This is a group of movements which depend 

 equally on both and are necessarily more complex than either. 

 Paired chromosomes during the pachytene stage express the laws 

 of both internal and external mechanics, which are combined in 

 the attainment of the special result of their relationship at this 

 stage, crossing-over between them. This, and the mechanics of 

 structural change of which it is a special form, may be dealt with 

 in a special class of more speculative considerations as ultra- 

 mechanics. 



In order to analyse the movements assigned to these categories 

 it is necessary to describe them according to a recognised canon of 

 mechanical nomenclature. For the present elementary treatment 

 it is easiest to follow the conventions of classical mechanics and 

 attribute movements to forces, whether of attraction, repulsion or 

 torsion. The student who finds this treatment objectionable is free 

 to translate these shorthand expressions into terms of energy 

 levels, but such a description is roundabout for our present purposes, 

 except in regard to the especially complex spindle system. We have 

 in the cell the same grounds for inferring forces that Newton had 

 in the universe : movements and accelerations of bodies regularly 

 occurring and accurately measurable. We use the same means of 

 inference : comparison of the movements and equilibrium positions 

 of bodies of different sizes and at different distances apart. 



In attempting to establish a new and closed system of mechanics 

 it is of the highest importance that we should begin our induction 

 from the right group of data, taking by analogy the right assumptions. 

 This is a matter of trial and error, and many false starts have been 

 made in consequence. The sound basis of external mechanics lay 

 in Lillie's observations in 1905, but this could not be used until 

 meiosis and mitosis were correlated. The sound basis of internal 

 mechanics was not revealed until Kuwada and Nakamura's experi- 



