HYSTERESIS 483 



merits showed the way. The soundness of these beginnings has 

 been shown only by their comparison with large bodies ot related 

 observations. How this has been done we shall now try to see. 



2. INTERNAL MECHANICS OF THE CHROMOSOMES 

 (i) The Problem. The series of changes undergone by the 

 chromosomes in the course of mitosis consist of a successive coiling 

 and uncoiling. From the point of view of their internal mechanics, 

 mitosis is therefore a spiralisation cycle. In the prophase each 

 chromatid develops an internal spiral as it shortens and thickens. 

 This is to be inferred from the structure observed at metaphase, 

 for within the prophase nucleus it cannot yet be differentiated. At 

 telophase in living cells the successive spirals can be seen to separate 

 and in the following prophase their relic spirals gradually uncoil 

 while the new internal spirals are being developed within each of 

 the chromatids into which the chromosome has divided. These 

 two chromatids themselves show a relational spiral which is, 

 however, facultative, for the chromatids of ring chromosomes are 

 usually able to separate and must therefore have been lying parallel. 



From this series of events four mechanical principles may be 

 inferred (D., 1935 a) : 



First, the coiling and uncoiling of the internal spirals must be 

 immediately determined by internal changes, for if they were exter- 

 nally determined, differences would always be found between 

 chromosomes of different lengths (Kuwada, 1935). The agent 

 must be torsion within the chromosome thread in the opposite 

 direction to the internal spiral. This may be described most simply 

 as due to the formation of a molecular spiral, a spiral torsion within 

 the thread. 



Secondly, the delay in uncoiling which leads to one cycle over- 

 lapping the next must be due to a failure of the external form of 

 the chromosomes to respond immediately to the internal stresses 

 due to changes in their molecular spiral. Such a hysteresis is clearly 

 due to the chromosomes lying in a somewhat rigid medium and 

 within a limited space, the resting nucleus. 



Thirdly, the relational coiling of chromatids must depend on the 

 specific attractions of their parts, for otherwise the chromatids 



16—2 



