The external mechanics of the chromosomes I—The scope of enquiry

Abstract

The movement of chromosomes may be regarded in two kinds of relationships according as it involves changes of shape and changes of position. The first are due to movements within the chromosomes, and may be used to infer their internal mechanics . The second are due to movements between chromosomes, and may be used to infer their external mechanic . Many experiments have been devoted to elucidating the principles of the external mechanics, and they have been successful in showing certain essential properties of the cell outside the nucleus, particularly of the spindle and the spindle-determining bodies or centrosomes. But, when applied to the chromosomes, artificial treatment has the drawback that in making one primary change it sets up a series of secondary changes whose importance cannot be accurately assessed; comparison is therefore vitiated. The cytoplasm and, in the resting nucleus, a semi-permeable nuclear membrane separates and protects the chromosomes from external stimuli. Thus micro-dissection and etherization have clear-cut effects on chromosome movements, but these may be secondary consequences of an action upon the cytoplasm. The value of experimental tests being thus limited, we are thrown back on the comparison of the behaviour of chromosomes in different natural circumstances. The number of variables is then under genetical control and the method of inference depends on genetical assumptions. But the necessary assumptions are now fairly well defined. We take it that the behaviour of the chromosomes depends on three variable factors: genotype, environment, and permanent structure, and there are usually means by which we can eliminate differences due to two of these. The characteristic differences between different species and varieties, for example, are due to differences of genotype, and they fall for the most part within a narrow range, the narrower the smaller the systematic group concerned. Even the most significant change that has arisen in the evolution of the genetic mechanism, the change from mitosis to meiosis, can be represented as the consequence of a shift in the time-coordination between external and internal factors in the development of the chromosomes, a precocity of the one relative to the other.