Chap. 3 LIVING MATTER AND CELLS 39 



Each pair of chromonemata was formerly a single thread (potential chromo- 

 some) with genes arranged along its whole length. As a thread doubles, 

 each gene makes a duplicate of itself out of materials lying close to it. As 

 a result of this a new string of genes, forming a new thread, lies close to 

 the old one and is identical with it, gene for gene, in every part (Fig. 3.8). 

 This creation of new genes, as pointed out by H. J. Muller (1947), "should 

 perhaps be regarded as the most remarkable process in nature; it consists 

 of the simultaneous creation, under the guidance of each gene, of a new 

 gene in its own image, lying next to itself and built out of materials lying 

 around it" (Fig. 3.8). Now having the layout of its future chromosomes, 

 each with its quota of genes, the nucleus is ready for reproduction. 



Reproduction of Body Cells — Mitosis. Cell division usually includes that 

 of the nucleus and cell body. However, the nucleus may reproduce when the 

 cell body does not and a multinucleate cell results. The cause of cell division 

 is not understood. If it were, the cause of cancer would be known, since 

 that is a disease of too rapid and usually abnormal cell division. 



Mitosis is the almost universal method of cellular reproduction. The only 

 significant exception is the variation of it called meiosis which occurs regu- 

 larly in the multiplication of sex cells. Mitosis is the precise rearrangement, 

 doubling, and separation of nuclear material by which two new nuclei are 

 formed that are quantitatively and qualitatively similar to each other and 

 to the nucleus from which they came. By means of it each daughter nucleus 

 receives an equal share of every substance which was in the parent nucleus. 

 It is a continuous process having four main stages; each stage has its own 

 characteristics but each merges into the one following (Fig. 3.8). 



Prophase (Preparation). Features of the interphase gradually change. 

 The knotted chromonemata are more distinct with the members of each 

 pair clinging together. At first each pair forms an irregular open spiral. 

 Then the coil tightens, shortens, and is filled in with darkly staining sub- 

 stance finally forming a chromosome. At the same time the centrosome just 

 outside the nucleus is active. It divides, and, if the cell has two centrioles, 

 they move toward opposite poles of the nucleus. If there was but one centriole 

 during the interphase, it now divides and the two new ones move apart. In 

 either case the area between them contains lines of protoplasmic particles. 

 These form the mitotic spindle, a double cone that at first lies a little outside 

 the nuclear field and later extends directly across it. This region is now occu- 

 pied by the chromosomes among the lines of the spindle and directly between 

 its dynamic poles. The nucleolus may still be visible, but it looks soft as its 

 substance begins to diffuse, seeming to scatter. 



Metaphase (Midway). The chromosomes are balanced midway between 

 the poles of the spindle (Fig. 3.8). Each one of the two chromonemata in a 

 chromosome has at exactly the same level a special point (centromere) of 



