CELL DIVISION ' 57 



lines are stretched, and around each one radiating Hnes may develop, 

 giving the appearance of a star. The contraction of the chromatin 

 continues, and before the process is more than well under way the chro- 

 mosomes are distinguishable as separate bands or ropes (C). The 

 entire scattered chromatin of the interphase nucleus is now collected 

 into these conspicuous bodies. Though the chromosomes have been 

 separate bodies all the time, it can now be seen for the first time that they 

 are distinct. While this change in the chromatin has been taking place, 

 the nucleolus, if one was present, has disappeared. The centrioles have 

 moved around to opposite sides of the nucleus, and very distinct threads 

 from them appear to be pushing against or even into the nucleus. The 

 membrane of the nucleus then dissolves away, leaving the chromosomes 

 free in the general protoplasm. Some of the threads from each centriole 

 quickly pass through the space formerly occupied by the nucleus and 

 connect with the other centriole, establishing a complete spindle between 

 them. Other threads go only halfway and end at the chromosomes. 

 The chromosomes shorten still further and thicken to form definite 

 bodies, often of very different shapes and sizes within the same cell. 

 The chromosomes are placed where the nucleus was, without any par- 

 ticular arrangement. The changes so far described, including stages 

 B to D in the figure, are collectively called the prophase, though the 

 plural form would be more accurate. 



Metaphase. — The chromosomes then move, probably are drawn, into 

 a flat group across the middle of the spindle. In this position they 

 form what is called the equatorial plate. Seen from the side of the 

 spindle they appear as in E, but viewed from one of the centrioles they 

 are as in F. This stage of mitosis is called the metaphase. It is of very 

 brief duration, so that it appears less often in preparations than the 

 other stages do. Either in the metaphase or at some earlier time, 

 the chromosomes become double structures. This doubling is usually 

 described as a division, but it may equally well be conceived as a dupli- 

 cation, that is, the formation of a second chromosome just like the 

 original. It is not important to decide at this point which of these 

 methods is employed, since in either case two identical chromosomes 

 exist where only one of that kind existed before. The chromosomes 

 are shown thus duplicated in E, less clearly so in F because of the direction 

 from which they are viewed. 



An important feature of this division is that the two chromosomes 

 produced from one are, in all significant features, identical with each 

 other and with the original chromosome which produced them. To 

 understand this fact one must know that the chromosomes have a 

 longitudinal pattern. They contain different substances at different 

 points in their length. A longitudinal division of the chromosome 



