SOMATIC CELL-DIVISION 107 



accompanies the more stainable chromonema at all times. Although it 

 may sometimes be distinguishable from the surrounding karyolymph 

 rather early, it ordinarily becomes conspicuous only when the prophase 

 is more advanced. 



The chromosomes, each composed of two chromonemata and sur- 

 rounding matrix, now become markedly thicker and often shorter. In 

 many nuclei this involves a contortion of the chromonemata, which then 

 appear to be crowded or coiled within the translucent matrix. At this 

 period the matrix develops a strong affinity for the stains commonly 

 employed, so that the whole chromosome may look like a solid body with 

 no internal structure. As the prophase draws toward a close each chrom- 

 osome tends gradually to assume its final form, which may differ charac- 

 teristically from that of the other chromosomes of the group. At this 

 time the nucleolus commonly disappears, although a portion of its sub- 

 stance may sometimes persist through the subsequent phases. 



At the end of the prophase a marked change occurs in the other 

 constituents of the nucleus. The nuclear membrane commonly dis- 

 appears, and the clear karyolymph in which the chromosomes are lying 

 develops a polarized arrangement which appears in fixed preparations 

 as a series of striations or fibrils lying parallel to the longitudinal axis 

 of the developing "mitotic figure." This mass of modified karyolymph 

 rapidly assumes the form of a bipolar spindle and the double chromo- 

 somes group themselves in its middle region. 



In the metaphase every chromosome lies at least in part in a well- 

 defined equatorial plane through the spindle. If the chromosomes are 

 very short, they may lie wholly in this plane; if they are long, all but a 

 small portion of each one may lie with no regular arrangement in the 

 figure. The portions lying in the equatorial plane are definitely deter- 

 mined: each chromosome has a specially differentiated and constantly 

 located "attachment region" at which it manifests a peculiarly intimate 

 connection with the spindle in the equatorial plane. Hence in the meta- 

 phase and immediately succeeding stage a given chromosome usually 

 exhibits clearly its characteristic morphology; the attachment region is 

 indicated by its relation to the spindle and by other features so that the 

 relative length of the "arms" on either side of this region can be easily 

 determined. At the attachment region the two longitudinal halves of 

 each chromosome lie facing the two poles of the spindle; in other portions 

 of the chromosome they often show no such orientation (Fig. 55). In 

 fixed preparations the most strongly developed "spindle fibers" extend 

 poleward from the attachment regions. 



In the anaphase the two halves of each chromosome move apart and 

 pass through the spindle toward its two poles, the movement beginning at 

 the attachment region. The shapes assumed by the various chromosomes 

 during this phase depend upon the location of the attachment region and 



