PROPHASE 25 



1921 ; and v. Ch. IV). The linear arrangement of the chromomeres 

 in different nuclei is constant. It seems unlikely that these delicate 

 structures are uninjured by fixation ; no doubt their discontinuity 

 is exaggerated. But their constancy shows them to be significant, 

 as well as characteristic, artefacts {v. Appendix I). It is therefore 

 difficult to regard them, as Kaufmann has suggested (1931), merely 

 as twists in the chromatid. They demonstrate morphologically the 

 linear differentiation of the chromosome. And it is important to 

 notice that the two chromatids correspond exactly. Since, as we 

 shall see, they must be supposed to arise from the longitudinal 

 division of one, that division has been, so far as cytological evidence 

 can show, into two identical halves. This identity is preserved 

 from the time the doubleness is first seen at prophase throughout 

 all stages of mitosis and all succeeding divisions of the chromatids 

 at succeeding mitoses. 



Further linear contraction of the chromosomes is accompanied by 

 an increase in their thickness. There is no direct evidence as to 

 what change takes place in the internal arrangement of the 

 chromosomes, but indirect evidence suggests that it is due to the 

 gradual assumption of a spiral form by the chromatid (v. infra). 

 The increase in thickness obliterates the distinction between 

 chromomeres and the chromatids gradually assume a rounded 

 outline and eventually a highly uniform cylindrical structure, apart 

 from constrictions, which will be considered later. 



In most organisms contraction of the chromosomes reaches its 

 maximum at the last stages of prophase. They are now probably 

 one-tenth to one-twentieth of the length they were at the earliest 

 stage of prophase. The two chromatids of each chromosome lie 

 parallel or slightly coiled round one another and in close proximity. 



(iii) Metaphase : the Spindle. At the end of prophase the 

 chromosomes attain, as a rule, their greatest contraction. A new 

 structure, the spindle, now develops. It varies in its method of 

 origin. It may arise inside the nucleus ; or when, as is usual in the 

 higher organisms, the nuclear-cytoplasmic surface breaks down, it 

 merely fills the space previously occupied by the nucleus. The 

 spindle also varies in different organisms in its shape and in its 

 appearance when treated with different reagents, but it can be 



