THE GENERAL OUTLINE OF MITOSIS 9 



every reason to believe that fixed and stained prepara- 

 tions give us an almost perfect picture of the appear- 

 ance in the living state. In the majority of nuclei 

 (all those which have not got prochromosomes) the 

 fixability is zero during the resting stage — thus the 

 first sign of prophase is the appearance of visible 

 threads (chromosomes) in the nucleus in place of the 

 network which results from the fixation of a resting 

 nucleus. 



The ' fixability ' of early prophase nuclei varies a 

 good deal, but appears to be correlated with the 

 volume of the nuclear sap in which the chromosomes 

 lie ; thus in the first spermatogonial division of the 

 grasshopper Metrioptera the nucleus is large and the 

 prophase chromosomes very ' fixable ' ; in the suc- 

 ceeding divisions the nucleus gets progressively 

 smaller and the chromosomes less fixable until the 

 sixth division when there is a sudden increase in the 

 size of the nucleus which is accompanied by an 

 increase in fixability ; the seventh and eighth 

 divisions again show small nuclei in which the early 

 prophase chromosomes fix badly. ^^^ 



The question now arises : what is the physical 

 basis of this property of fixability? There is con- 

 siderable evidence that it depends on the degree of 

 colloidal hydration of the chromosomes. Belar ^^ 

 showed by experiments on the dehydrating power of 

 hypertonic solutions on the nucleus that the meta- 

 phase chromosomes contain less water than any other 

 nuclear constituent and we have reason to believe 

 that the resting-stage chromosomes are highly 

 hydrated, so one must infer that dehydration is one 

 of the processes involved in prophase. 



In all cases the chromosomes at the very earliest 

 prophase are separate ; there is thus no ' continuous 

 spireme ' as described by some of the earlier cytolo- 

 gists (the oogonial nuclei of the scale insect Icerya 

 purchasi form a possible exception, ^^ but the case 

 needs re-investigation). Further, the individual 



