MEIOTIC PHASE IN MARSUPIALS 195 



produced by the action of the fixative on a homogeneous 

 thread, but the view that the chromosomes are composed of 

 differentiated chromomeres cannot be disposed of by demon- 

 strating mistaken interpretation in individual cases. On the 

 contrary, we have now a large accumulation of observations 

 where the answer to the above question seems undoubtedly. to 

 be in the negative ; observations, that is to say, which lead 

 to the conclusion that whether the beads exist as such in hfe, 

 or whether they are produced by the fixative at the moment of 

 death, they must be expressions of local differentiations of the 

 substance of the chromosome — and that is all, of course, that 

 is required by the theory that connects the chromomeres with 

 the linear arrangement of hereditary factors in a chromosome. 

 The reasons which seem to exclude the view that such chromo- 

 meres are produced mechanically, and so to speak, accidentally, 

 on a homogeneous thread which is contracting unequally under 

 the influence of unequal stresses in different parts are : 



(1) The chromomeres in a single chromosome may differ 

 very greatly as regard size (PI. 13, figs. 27, 28). 



(2) Had they been produced by unequal contraction of parts 

 of a homogeneous thread, larger chromomeres would be 

 separated from each other by longer intervals of connecting 

 thread than those which separate the smaller chromomeres. 

 A glance at fig. 28 show^s that this rule does not hold. 



(3) There is a close correspondence between the chromomeres 

 of homologous chromosomes, both during syndesis and the 

 diplotene stage. 



(4) Wenrich (1916) has described the constant arrangement of 

 the principal chromomeres on a given chromosome — a con- 

 stancy which is maintained not only in all the nuclei (of the 

 same stage) in a single animal, but even in different animals. 

 Unless this observation be doubted, it supplies conclusive 

 evidence that the chromomeres as seen in fixed nuclei corre- 

 spond to definite local differentiations of the substance of the 

 chromosome. 



(5) It is now well known that the shape assumed by the long 

 type of chromosome common in many forms of mitosis is 



