34 MITOSIS : THE CONSTANCY OF THE CHROMOSOMES 



which in the longest chromosomes is still seen at the following 

 metaphase. Whatever internal forces cause the coiling and un- 

 coihng we see that there is a lag in the adjustment of the external 

 form of the chromosomes to the changes in these forces : the 

 coiling seen in a prophase is a relic of that at the preceding division, 

 not the forerunner of that at the following division, as has been 

 generally imagined. In some respects, to be sure, prophase reverses 

 telophase. In other respects prophase continues telophase. 



The changes inferred in the transition of the metaphase chromo- 

 some to the telophase condition have now been successfully imitated 

 in experiment. Kuwada and Nakamura (1934 a) have changed 

 the body of metaphase chromosomes in a pollen mother-cell into a 

 resting nucleus by exposing them to ammonia vapour before fixation. 

 The effect is to half-uncoil the spirals so that they form a compact 

 mass having the optical properties of a resting nucleus that has 

 been similarly fixed with acetic acid. The artificial differ from the 

 natural nuclei merely in their irregularity of boundary and in their 

 lack of nucleoli {cf. Ch. XII). 



These observations show that the resting nucleus and metaphase 

 chromosomes, which appear equally homogeneous in life and are 

 equally susceptible to the production of non-characteristic artefacts 

 through salting-out of their protein- water systems, are in fact, to 

 be related in terms of a spiral structure which can be traced through 

 successive stages in life and reproduced artificially in experiment. 

 The change from metaphase to resting nucleus Consists spatially in 

 uncoiling and re-packing the chromosome thread. 



(ii) Constrictions, Centric and Nucleolar. In the course of 

 })rophase another kind of discontinuity appears in the chromatin 

 thread in many organisms. This is at first an unstained gap in 

 the chromatid ; afterwards it develops into the metaphase 

 " constriction," at which the cylindrical body of each chromatid is 

 drawn together as a sausage is drawn together with string or a 

 blood vessel ligatured. All chromosomes have one such non- 

 spiralising segment in their structure — the centric constriction. 

 But there are also found in many chromosomes " secondary " 

 constrictions which have no relationship with the centromere 

 (cj. Plate II and Fig. 7). 



