THE GENERAL OUTLINE OF MITOSIS 25 



membrane of the daughter nuclei during telophase 

 are rather complicated hut may ])riefly he described 

 as a reversal of those which take place during the 

 latter half of prophase — that is to say, de-condensa- 

 tion and de-spiralization. As a result of the latter 

 the chromosomes become elongated and thrown into 

 tight zig-zags inside the nuclear membrane. As 

 they pass into the resting stage they become once 

 more hydrated and lose their ' fixability '. 



It will be remembered that we described the early 

 prophase chromosomes as longitudinally split or 

 divided into two chromatids. The telophase chromo- 

 somes, on the other hand, are unsplit and thus 

 consist of a single chromatid each. The division of 

 the chromosome in preparation for the next division 

 thus takes place during the resting stage.* 



The whole process of mitosis usually takes several 

 hours from start to finish. From 2-24 hours is 

 probably the usual range of variation in most organ- 

 isms. In special cases, however (see Chap. I), it 

 may take much longer. Prophase is nearly always 

 the longest stage, prometaphase, metaphase and the 

 two parts of anaphase are all short stages, while 

 telophase is considerably longer, but usually not so 

 long as prophase. 



To sum up : mitosis consists of a series of cyclical 

 colloidal phenomena, each of which is reversible. 

 The main ones, so far as the chromosomes are con- 

 cerned, are hydration, de-hydration, spiralization 

 (condensation) and de-spiralization (de-condensation). 

 Growth of the chromosome substance is probably an 

 irreversible process and leads to the formation of two 

 chromosome sets from what was originally a single 



* There may be exceptions to this. Thus it appears that in 

 anaphase and telophase of the second meiotic division in the 

 plant T radescantia the chromosomes are already split in 

 preparation for the next division.'' But in general the 

 division of the chromosomes would appear to take place during 

 the resting stage. ^i'- ^^^^ ^^^ 



