The internal mechanics of the chromosomes I—The nuclear cycle in Fritillaria

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Abstract
The nuclear cycle in plants and animals consists in the alternation of two mechanically stable systems, the resting nucleus and the metaphase chromosomes. The regularity of this alternation and the constancy of the bodies taking part in it has led to the assumption that it depends on a “permanence” of the positional relationship of essential elements in these bodies, that is, on a property of passing on the same structure from mother to daughter nucleus and from mother to daughter organism by the structure reproducing its like without change. This assumption has been vindicated by the demonstration at a particular nuclear division, meiosis, in the transitional and generally unstable stages of prophase between the resting nucleus and the metaphase chromosomes, of a linear arrangement of particles which is constant both in individuals and in races. The inference of this constant arrangement of particles depends on evidence of form and of function. When the inference is from form the particles are chromomeres , when the inference is from function rezealed by mutation, by crossing-over, or by specific attraction, the particles are genes . The first problem in considering the nuclear cycle is therefore to discover the spatial relationship between the mechanically unstable prophase threads which reveal this exact and indisputable property of organization, viz., linear arrangement, and the stable resting nucleus and metaphase chromosomes, which are optically homogeneous in the living condition and whose properties of organization as revealed by treatment are disputable. We must find how the one changes into the other. The problem is dynamic not static. Its solution must depend on observations of the natural building up of the chromosomes in the prophase of mitosis, or of their natural or artificial breaking down in the telophase. It must be related to our knowledge of the permanence of the linear arrangement of particles at all stages and of their reproduction at one stage in the cycle. This study I would describe as the study of the internal mechanics of the chromosomes, that is of the principles governing their internal stresses and adjustments. I would thus distinguish changes within the chromosomes from the class of changes with which exact study has hitherto been chiefly concerned, namely, those such as pairing and terminalization, orientation, and disjunction, which affect the relationships of different chromosomes with one another and with other cell-elements. The principles governing these much simpler and clearer changes I would describe as the external mechanics of the chromosomes. The distinction between the two studies is arbitrary but convenient for analysis. They meet when we come to consider the mechanism of crossing-over. Together they must aim at explaining in common terms every change, no matter how trivial or how important, in the size, shape, position, and attitude that makes up the visible life of the chromosomes.