12 THE CHROMOSOMES 



fourth — ' despiralization '. That is to say that as 

 shortening and thickening take place the spirals of 

 the early stages unwind. ^^ 



In well-fixed chromosomes it is possible to see 

 from the very beginning of prophase that the staining 

 substance of the chromatids is not continuous from 

 end to end ; it is interrupted at one point at least 

 (in plantr chromosomes usually several points) to 

 form a non-staining gap (Fig. 2). ^ These gaps become 

 more obvious later, and are called constrictions : 

 their position is constant for each chromosome. 

 They are filled by a non-staining substance which is 

 not nuclear sap and which holds the chromatids on 

 either side of it together. 



Throughout the prophase of mitosis the outlines 

 of the chromatids present a slightly irregular woolly 

 or hairy appearance which is probably an ai^tefact : 

 they do not in general show a series of granules 

 (chromomeres) such as are seen at the meiotic pro- 

 phase ; this may be a real difference and not due to 

 difference in fixability. By the end of prophase the 

 woolly appearance referred to above has almost 

 disappeared and a smooth outline has taken its 

 place. 



The long threads of the early prophase chromo- 

 somes appear to wind more or less at random through- 

 out the nuclear cavity ; but they never actually 

 come in contact with one another, or indeed approach 

 within a certain minimum distance : there is tlius 

 something which keeps them apart, which is probably 

 in the nature of a generalized electrostatic repulsion 

 distributed over the surface of the chromosome. As 

 prophase advances there is a tendency for the 

 shortened and thickened chromosomes to move to the 

 periphery of the nucleus and to arrange themselves 

 on the inner surface of the nuclear membrane. 



If a nucleolus or nucleoli are present in the resting 

 stage they usually lose their staining power during 

 prophase and have disappeared completely by pro- 



