THE REDUCTION OF THE CHROMOSOMES 



229 



of their substance. In the heterotypic mitosis, the first of the two matura- 

 tion mitoses, the chromosomes conjugate two by two during the prophase to 

 form the reduced number of bivalent chromosomes, which take their place 

 on the spindle. The members of each pair, which are supposed to differ 

 qualitatively from each other, separate and pass to the two daughter nuclei. 

 These nuclei are therefore qualitatively unlike each other, having different 

 members of the full chromosome group; and also unlike the mother nucleus, 

 since each of them has only half as many chromosomes as the latter. The 

 second maturation mitosis (not shown in the diagrams) is essentially a 

 vegetative mitosis in most cases: each chromosome splits longitudinally 



TEROTYPtC Ml 



FIG. 82. Diagram showing essential difference between somatic and hetero- 

 typic mitoses. 



and the halves are distributed to the daughter nuclei. The four nuclei, 

 and consequently the four cells, resulting from the two maturation divi- 

 sions are therefore of two kinds : two of them have half of the chromosomes 

 of the original nucleus and the other two have the remaining ones. 



Assuming, then, that the chromatin of the nucleus represents the 

 principal physical basis of inheritance (see Chapter XIV), reduction is 

 essentially this: a reduction in the number of kinds of hereditary units by 

 the separation and distribution of qualitatively different masses of chromatin 

 to different cells and eventually into different hereditary lines, rather than an 

 equational division and distribution of all the qualities as in somatic mitosis. 

 As has already been stated, the change in the number of chromosomes 

 ("numerical reduction") is a consequence of the method by which this 

 qualitative reduction is brought about, this method being the distribution 

 of entire chromosomes, each representing one or more particular qualities, 

 to different cells. 



