22 THE CHROMOSOMES 



division if the resting stage is short (if it is protracted 

 they may completely unwind before the beginning 

 of prophase). Darlington ^^ calls the early pro- 

 phase spirals relic spirals ; if in addition to the relic 

 spirals a larger spiral is present at early prophase, 

 he calls the larger one a super spiral, but both relic 

 and super spirals are due to the same cause, namely 

 unwinding of the previous metaphase spiral. 



There are three ways in which the metaphase 

 spiral might develop during late prophase and pro- 

 metaphase. There is, unfortunately, no direct evi- 

 dence as to which of these actually occurs, since 

 observations on the origin of the metaphase spiral 

 are very incomplete, the internal structure of the 

 chromatids being difficult to study at this period. 



According to the first method the chromosome 

 rotates in order to become spiralized (either one 

 end remains fixed and the other rotates or both 

 ends rotate in opposite directions or the spindle 

 attachment remains fixed and the ends rotate). 

 Darlington ^* has given several reasons why this 

 cannot be the mode of origin of the metaphase spirals. 

 According to the second method an internal compen- 

 sating spiral (whose twists are below the limit of 

 resolution of the microscope) develops in the opposite 

 direction to the main one (i.e. right-handed if the 

 main one is left-handed and vice versa) and with 

 the same number of turns as the main one. In 

 order to understand this internal compensating spiral 

 one can carry out a simple experiment with a piece 

 of copper wire : fix the two ends in a pair of vices 

 and wind the middle j)art into a spiral round a metal 

 rod : the compensating spiral will easily be seen. 

 Darlington believes in the existence of this com- 

 pensating spiral (which he calls the molecuktr spiral) 

 and regards it as causing the development of the 

 metaphase spiral. According to the third method 

 (which is possible in a colloidal body like a chromo- 

 some, but not in a piece of copper wire) there is no 



