THE CELL AND CELL DIVISION 51 



somatic cells. In all but a few groups the chromosomes appear 

 in an even number, ordinarily between twelve and thirty-six, 

 although these limits are frequently passed. The chromosomes 

 are at present considered the most important elements in the 

 cell, and interest in the whole process of mitosis centers in their 

 behavior. The details of the process of mitosis seem directed 

 toward the exactly equal division and distribution of these 

 elements, the importance of which justifies the more detailed 

 consideration which we give them after the general description 

 of mitosis is completed. 



While the chromosomes are forming, the centrosomes and 

 asters continue to diverge, passing around toward opposite sides 

 of the nucleus. The linin fibers of the nucleus tend throughout 

 to remain polarized toward the centrosomes and the separation 

 of these bodies from one another draws out the linin fibers into 

 an elongated bundle converging at each end toward the centro- 

 some. Finally the centrosomes come to lie on exactly opposite 

 sides of the nuclear structures and as the nuclear membrane 

 disappears completely we find the rays of the asters penetrating 

 into the nuclear region and forming, together with the linin, 

 a spindle-shaped structure lying between the centrosomes, its 

 component fibers passing among the chromosomes (Figs. 20, G; 

 21, F'j 22, D). In many cells the centrosomes do not thus 

 migrate to opposite sides of the nucleus, but separate directly; 

 the nucleus in this case is simply drawn up to lie between them 

 (Fig. 21, D). The result is the same, but the difference in 

 relative behavior of the centrosomes and nucleus is real and 

 must be taken into account in some cases. The spindle and 

 asters now form a figure resembling a diagram of the lines of 

 force within a simple bipolar magnetic field. This figure is 

 called the amphiaster, sometimes the achromatic figure, empha- 

 sizing its distinctness from the chromatic portion of the nucleus, 

 now all or largely included in the chromosomes. The parts of 

 the linin network directly continuous with those upon which 

 the chromosomes were formed originally, seem to have a some- 

 what different history from the remainder of the linin. They 

 remain attached to the chromosomes and extend thence toward 



