THE GENERAL OUTLINE OF MITOSIS 21 



methods of fixation, however (fixing in boiUng 

 water, ^^® squeezing the chromosomes under a cover- 

 glass, ^^ exposing them to fumes of ammonia or 

 strong acids ,^^' ^® it is possible to show that each 

 chromatid has a spiral structure, the apparent 

 cylinder being a spring in which the successive 

 gyres are in contact (Fig. 2d). There is no doubt 

 that this is the true structure in the living state ; 

 all that the special methods of fixation have done is 

 slightly to separate the gyres and thus reveal the 

 spiral. The two chromatids are coiled independently 

 (in other words their gyres do not interlock as 

 happens when two parallel wires are wound round a 

 cylinder) and in the same direction (right- or left- 

 handed) at any one level ; the direction of coiling 

 may change at the spindle attachment, but does not 

 necessarily do so ; there is considerable doubt 

 whether it may change direction elsewhere. 3*' '^^^ ^^^' ^'' 



The existence of a spiral structure in metaphase 

 chromosomes was discovered as early as 1880 by 

 Baranetsky and there seems no doubt that it is 

 universally present both in plants and in animals. 



The occurrence of a metaphase spiral explains the 

 contract and condensation process during prophase ; 

 this must now be interpreted as due to the develop- 

 ment of the metaphase spiral. It will be remembered 

 that in nuclei whose resting stage is of short duration, 

 the early prophase chromosomes are also spiralized. 

 We have therefore two kinds of spirals, those of early 

 prophase and those which develop at the end of 

 prophase and are completed by metaphase. The 

 metaphase spirals are, however, not a continuation 

 of the early prophase ones, since the latter have 

 disappeared by mid -prophase ; as a matter of fact 

 the reverse is the case ; that is to say, the early 

 prophase spirals are the remains of the metaphase 

 spirals of the previous division which have persisted 

 through the intervening resting stage, and only 

 finally unwind in the mid-prophase of the next 



