THE LIVING ORGANISM 221 



material of the nucleus then forms itself 

 into a thread-like structure so as to look like 

 a tangled skein of thread, which later shortens 

 and thickens. Then in a most remarkable way 

 the thread divides or becomes cut up into a 

 definite number of short lengths, called 

 chromosomes. The number of these chromo- 

 somes is an absolutely fixed and always an 

 even number in every species of higher animal 

 or plant. In man there are, for example, 

 always twenty-four of these chromosomes 

 formed. The chromosomes were formed by 

 equi-distant cuts across the skein, but now 

 each one of these pieces splits lengthwise, and, 

 at about the same time, two attraction points 

 are formed some distance apart in the cell, 

 and from these attraction points (or centro- 

 somes) exceedingly fine gossamer-like threads 

 are formed radiating from the centrosome, 

 and attached along the length of each split 

 chromosome. Along these fine threads the 

 split halves of the chromosomes part company 

 from each other, and travel apart from each 

 other, until finally one exact half of the 

 material of the nucleus in the form of split 

 chromosomes is surrounding each centrosome. 

 An inverse order of affairs now ensues at 

 each of these daughter nuclei, whereby first 

 a skein and then a reticulum is formed. 



