THE GENERAL OUTLINE OF MITOSIS 11 



chromosomes are always double from the very 

 begimiing of prophase, with the two threads or 

 chromatids of which they are composed closely 

 approximated throughout their length. Where there 

 are size differences between the chromosomes the 

 ratio of the lengths at early prophase is approxi- 

 mately equal to the ratio of the volumes at meta- 

 phase and it is possible to pick out the pairs of 

 chromosomes in a diploid organism. 



The appearance of early prophase chromosomes 

 depends to a certain extent on whether the preceding 

 resting stage has been of long or short duration. 

 Where it has been short and the chromosomes are 

 few in number as in the cleavage divisions of the 

 Horse Roundworm, Ascaris megalocep?iala, it is 

 possible to show ^' that the individual chromosomes 

 become visible in the same positions as they had 

 disappeared in at the previous telophase. Again, 

 where the resting stage has been short and the 

 chromosomes are relatively long and * fixable * it 

 can be seen that at the early prophase stages they 

 are coiled into loose spirals (Fig. 2a). Frequently 

 each chromosome is not merely a * spring ', but this 

 spring itself describes a wide open spiral in the nuclear 

 cavity ; in this case the smaller spiral has perhaps 

 5-20 turns, the larger one lJ-2 turns. It appears 

 that these two spirals are always in the same direc- 

 tion in a single chromosome, left- or right-handed, 

 as the case may be.^* The two chromatids are 

 always closely approximated from one end of the 

 spiral to the other. 



As prophase proceeds the volume of the chromo- 

 Bomes increases considerably ; there is thus an actual 

 manufacture of new material during prophase. 

 Side by side with this a shortening and thickening 

 of the chromatids takes place. We have thus 

 identified three processes which are involved in the 

 development of prophase : dehydration, growth, and 

 condensation or contraction ; we must now add a 



