The external mechanics of the chromosomes VI—The behaviour of the centromere at meiosis

In an attempt to analyse the forces governing the movements of chromosomes during division, variations from one cell to another and from one organism to another provide the only satisfactory method we know of determining the variables concerned, in short, of defining a “normal”. By methods such as these it has been established that the same forces act in mitosis as in meiosis, and are merely co-ordinated differently. In meiosis the co-ordination is more complex and delicately balanced than in mitosis. It is therefore more easily disturbed, and its variations are more frequent and of many different kinds. Moreover, even if lethal to their products, they are not lethal to the organism, as even slight variations of mitosis usually are; hence they are more easily studied. Each of these kinds of variation may give a clue to some problem which in the “normal” remained obscure or unnoticed. Each is a natural experiment which, while not usually disturbing the process so far as to cause death as man-made experiments often do, leads us by comparison to a further understanding of the normal. The variations may be of several kinds. They may be determined by the possession of an abnormal genotype (Darlington and Thomas 1937), or by structural or numerical changes in the chromosomes. They will affect all the cells undergoing meiosis although perhaps in varying degrees. A further set of variations consists in temporary and environmental abnormalities affecting single cells or groups of neighbouring cells.