26 MITOSIS: THE CONSTANCY OF THE CHROMOSOMES 



recognised everywhere by certain general characteristics. It lies 

 between two more or less defined poles, and has more or less regularly 

 the shape from which it takes its name. It is more rigid than the cyto- 

 plasm surrounding it and has less water to lose. When dehydrated 

 in fixation it contracts crosswise more than lengthwise and frequently 

 shows a lengthwise striation when contracted in this way. It is 

 therefore evidently differentiated in its water content, which must 

 be arranged in longitudinal channels to some of which the name of 

 " spindle fibres " probably refers. Outside the nucleus, in most 

 animals and in many of the lower plants, lies a body which is 

 actively concerned in the division of the nucleus. This is the 

 centrosome. It consists of a small granule which is sometimes 

 capable of differential staining. It divides into two during 

 telophase or prophase. There has often been difficulty in following 

 its history, but it would appear from Boveri's studies that it is 

 transmitted solely by the sperm of which it constitutes the " middle- 

 piece," the centrosome of the egg-cell degenerating in sexual 

 reproduction. Where centrosomes are present the spindle nearly 

 always develops between them as an extension of a radial differentia- 

 tion of the cytoplasm which appears around them during prophase. 



During the prophase the chromosomes have been disposed 

 evenly throughout the nucleus. As the spindle envelops the 

 nucleus and its boundaries disappear the chromosomes come to lie 

 midway between the two poles of the spindle in a flat " plate " on 

 which they distribute themselves evenly. Comparison of different 

 organisms shows that this distribution depends on one point in each 

 chromosome lying on the equator of the spindle. This point 

 of association with the spindle, or point of " attachment " to it, 

 as it has been somewhat misleadingly described, is marked by a 

 constriction or hiatus in the chromosome. This constriction marks 

 the position of the centromere. This body is usually unstained at the 

 metaphase of mitosis, and its behaviour will be described in detail 

 later (Ch. XII). The constriction may be described as the centric 

 constriction. 



The parts of the chromosomes near the centromere are spoken 

 of as " proximal," those further away as " distal." Where the 

 chromosomes are short they may lie entirely in the equatorial 



