THE CELL AND PROTOPLASM. 99 



which is the most probable, they are pulled or 

 pushed around by the spindle fibres which are 

 forcing their way into the nucleus, is not posi- 

 tively known ; nor is it, for our purposes, of special 

 importance. At all events, the result is that when 

 the asters have assumed their position at oppo- 

 site poles of the nucleus the chromosomes are ar- 

 ranged in a plane passing through the middle of 

 the nucleus at equal distances from each aster. 

 It seems certain that they are pulled or pushed into 

 this position by forces radiating from the centro- 

 somes. Fig. 30 shows this central arrangement 

 of the chromosomes, forming what is called the 

 equatorial plate. 



The next step is the most significant of all. 

 It consists in the splitting of each chromosome 

 into two equal halves. The threads do not divide 

 in their middle but split lengthwise, so that there are 

 formed two halves identical in every respect. In 

 this way are produced twice the original num- 

 ber of chromosomes, but all in pairs. The pe- 

 riod at which this splitting of the chromosomes 

 occurs is not the same in all cells. It may oc- 

 cur, as described, at about the time the asters 

 have reached the opposite poles of the nucleus, 

 and an equatorial plate is formed. It is not in- 

 frequent, however, for it to occur at a period con- 

 siderably earlier, so that the chromosomes are 

 already divided when they are brought into the 

 equatorial plate. 



At some period or other in the cell division 

 this splitting of the chromosomes takes place. 

 The significance of the splitting is especially note- 

 worthy. We shall soon find reason for believing 

 that the chromosomes contain all the hereditary 

 traits which the cell hands down from generation 



