226 FOUNDATIONS OF BIOLOGY 



At the beginning of the prophase, or earlier, the centrosome 

 divides to form two, each of which becomes surrounded by 

 what appears to be a halo (ASTER) of radiating fibers which 

 are possibly cytoplasmic currents the visible expression of 

 physico-chemical forces. The centrosomes and asters now 

 proceed to move apart, take up positions at opposite sides 

 of the nucleus, and the astral fibers between lengthen until 

 they form a CENTRAL SPINDLE. While these changes are 

 going on, the nucleus is not inactive. The nuclear membrane 

 gradually disappears and the chromatin granules, originally 

 in a net-like arrangement, seem to become rearranged in a 

 more or less continuous thread of chromatin called the 

 SPIREME. This, however, actually represents a number of 

 definite chromatin entities, termed CHROMOSOMES, which 

 gradually by chromatin concentration become distinctly in- 

 dividual. The number of chromosomes varies greatly in 

 different species, but is typically an even number and the 

 same for all the cells of a given species. 



When the chromosomes have assumed definitive form, the 

 preliminary events which constitute the prophase of mitosis 

 are brought to a close by the chromosomes being drawn to 

 the center of the spindle. Here they are arranged in a plane 

 at right angles to the long axis of the central spindle, midway 

 between the two centrosomes, and form the EQUATORIAL 

 PLATE. 



And now the stage is set for what is apparently the climax 

 of mitosis, designated the metaphase. Each of the chromo- 

 somes separates into two parts along the line of a longitudinal 

 split, in such a manner that each of the thousands of chromatin 

 granules which make up a chromosome is equally divided. 

 Two sets of similar daughter chromosomes are thus formed. 



With chromosome division consummated, the metaphase 

 merges into the anaphase which is devoted to a shifting of a 



