THE CHROMOSOME MECHANISM 



Boveri, was that the chromosomes remained in being in the resting 

 nucleus. They remained as invisible threads, each of which split 

 lengthwise, or reproduced itself, to give two identical chromatids 

 which reappeared in the following mitosis. 



There are many ways of showing that the theory of the repro- 

 duction of chromosomes, which at first seemed unnecessary and 



Fig. 4. — Diploid chromosomes at metaphase of mitosis in polar view to show the 

 arrangement in a flat plate. Left, Drosophila melanogaster male with XY, two large, 

 and one very small autosome pair. Right, Crocus hyemalis with five pairs. All 

 chromosomes have centric constrictions and in addition one Crocus pair has a 

 nucleolar constriction. X 3,000. 



fantastic, is both necessary and true. The simplest way is by com- 

 paring the uncoiling threads of telophase with those which become 

 visible at prophase. Then we see the uncoiling still going on. There 

 are coils which straighten out and disappear only just before meta- 

 phase, and these are obviously relic coils, the outward and visible 

 signs of the inward and invisible coils of the preceding mitosis. As 

 we proceed step by step new evidence of this fundamental principle 

 of continuity will come to light. 



The Polytene Nucleus 



There is another kind of indication of what the chromosomes 

 are doing, or can do, in the resting nucleus in certain large gland 

 cells. Such nuclei, for example in the salivary glands of flies, con- 

 tinue growing without ever dividing. Their chromosomes, however, 

 remain stainable and divide again and again inside the nuclei. They 

 reproduce while continuing their ordinary work. Instead of the 

 double chromatids of prophase nucleus, there are then bundles of 



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