28 CELL DIVISION [CH. 



of the nucleus, the disappearance of the nuclear membrane 

 and internal network, the aggregation of the chromatin into 

 chromosomes, and the division of these into halves from 

 which finally two daughter nuclei are reconstituted. The 

 first change that appears in the nucleus when division is 

 approaching is that the chromatin, instead of being scat- 

 tered in fine granules over the network, begins to become 

 aggregated into larger masses, and at the same time it 

 seems to increase in amount, either in some cases by trans- 

 ference of material from the karyosome, or, possibly, as was 

 mentioned above, by the production of fresh chromatin 

 from the oxyphil (plastin) granules. The concentration of 

 the chromatin may take place in either of two ways ; it may 

 become aggregated into irregular masses on the nuclear net- 

 work, often just beneath the membrane, or the network 

 may resolve itself into a coiled thread or skein known as a 

 spireme, on which the chromatin granules are collected and 

 gradually grow into more concentrated masses. In the first 

 case after a time it is seen that there is a definite number of 

 centres about which the chromatin is being collected; the 

 masses become more compact, including all the previously 

 scattered granules, and finally take the form of rounded, 

 oval or rod-shaped bodies of chromatin known as chromo- 

 somes. The number of these is perfectly definite and, with 

 certain exceptions to be considered later, is constant, not 

 only for every cell in the individual animal, but for the 

 whole species. The number differs, however, very greatly 

 in different species, ranging from very few (four, six, or 

 rarely even only two) up to numbers approaching two 

 hundred, but the majority of animals have between twelve 

 and fifty. For reasons which will become clear later the 

 number of chromosomes is commonly an even one. In the 

 second method of concentration, when a skein or spireme is 



