THE CELL AND PROTOPLASM. 117 



nected by the spindle fibres. Meantime the 

 membrane around the nucleus has disappeared, 

 and thus the spindle fibres readily penetrate into 

 the nuclear substance (Fig. 30). 



During this time the chromosomes have been 

 changing their position. Whether this change in 

 position is due to forces within themselves, or 

 whether they are moved around passively by 

 forces residing in the cell substances, or whether, 

 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 opposite poles of the nucleus the 

 chromosomes are arranged 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 centrosomes. Fig. 30 

 shows this central arrangement of the chromo- 

 somes, 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 

 number of chromosomes, but all in pairs. The 

 period at which this splitting of the chromosomes 

 occurs is not the same in all cells. It may 

 occur, as described, at about the time the asters 



