CELLS IN DIVISION 



are grouped round the 'pole field', with the arms of the chromosomes 

 hanging parallel downwards. This arrangement corresponds to that of 

 the chromosomes during the late anaphase of the previous division, 

 and demonstrates the morphological continuity of the chromosomes 

 between one division and the next.* This must mean that the chromo- 

 somes remain immobile during this whole period, although there seems 

 to be some disarrangement of the chromonemata. 



The duration of prophase is four hours or more. There is no abrupt 

 transition to metaphase; the pole caps enlarge, the nuclear membrane 

 disappears and the chromosomes contract still further. Belar was not 

 able to follow the dissolution of the nucleoli. At 20° C. the interval 

 between late prophase and late anaphase is about one and a half hours. 

 The changes undergone by the chromosomes in telophase are apparently 

 the reverse of those seen in prophase; for gradually their spiral condition 

 becomes clearly evident, and is later obscured once again as the nucleus 

 assumes the interphase condition. The chromosomes shrink as they 

 approach their 'spiral telophase' stage; afterwards the chromonemata 

 loosen out and approach each other. Their parallel arrangement may 

 persist for the first hour or more of interphase, and the nucleoli do not 

 reappear until after the wall has been formed between the daughter 

 cells. 



Mitosis in living cells in other plant tissues was studied by Martens. ^^ 

 His technique was to cut thin free-hand sections of developing organs 

 and to mount them in isotonic saccharose ('Beaucoup de prudence et 

 de promptitude sont necessaires') ; observation of a single preparation 

 was limited to a maximum of 75 minutes. Martens compared the 

 appearance of the living cell at each stage of division with that of the 

 same cell on fixation, but he did not make parallel observations with 

 stained preparations. The material which Martens used for these 

 studies was the developing stigma of the grass Arrhenatherum and the 

 embryo of the orchid Listera (the Twayblade), in both of which the 

 nuclei differ in several respects from those of Tradescantia. In interphase, 

 their structure is reticular, and does not greatly alter in appearance 

 during early prophase. The meshes of the network are gradually 

 resolved into spiral threads which become the chromosomes; for a 

 long time they are linked by delicate filaments. In mid-prophase, the 

 volume of the nucleus increases ; both the nucleoli and the anastomoses 

 between the chromosomes then stand out more clearly, while the spirals 

 of the chromosomes become gradually less visible. 



In Arrhenatherum, there are comparatively swift changes at the end of 

 prophase. The cross-filaments between the chromosomes disappear, 

 and within twelve minutes or less, they begin to move into a parallel 



* Rabl^' first drew attention to such residual orientations of prophase in Amphibian cells 

 in 1885. 



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