CHAPTER I 



THE CHROMOSOME MECHANISM 



Mitosis The Polytene Nucleus Meiosis 

 Reduction and Recotiibination 



Mitosis 



The growth of the organism depends on the growth and multi- 

 pHcation (or, as we call it paradoxically, division) of its cells and 

 nuclei, which must evidently divide together if each cell is to have 

 its one nucleus. The process by which they divide is known as 

 mitosis, and it is during this process that we are able to study the 

 structure of the nucleus. 



The first sign of mitosis is shown by a change in the nucleus. In 

 the undividing cell the nucleus is said to be at rest. It is globular and 

 shows no structure apart from one or two dense storage bodies, the 

 nucleoli. When mitosis begins, dense and stainable threads appear 

 in a more fluid medium. These threads are double. They shorten and 

 thicken by forming a spiral, to give curved or bent double rods, the 

 chromosomes, which are released into the cell when the membrane 

 or boundary of the nucleus disappears. 



The release of the chromosomes marks the end o( prophase. At the 

 same time, the beginning of metaphase is shown by the appearance 

 of the spindle. The spindle is a liquid-crystal structure whose fibres 

 lie parallel to its axis. The chromosomes, now at their shortest 

 (25 n down to I /^ or less), come to lie in the wider middle of the 

 spindle and form a flat plate. We can then see five of their important 

 properties (Fig. 3). 



The first is that the number of chromosomes in the mitoses of one 

 individual, and often indeed of one whole species, is constant, 

 whether it be two thousand or only two. The second is that the 

 chromosomes are also constant in shape and relative size. The third 

 property is revealed by the second and is true of all cells in the 

 higher plants and animals except the germ cells: there are two 

 chromosomes of each kind or, if you like, two similar sets. The 



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