154 INTRODUCTION TO CYTOLOGY 



The material constituting the cross pieces gradually moves to the two 

 side strands, the center portion of the cross piece becoming progressively 

 thinner and the material accumulating on the side strands as a pair of 

 chromatic lumps. Although some of the cross pieces may persist until a 

 relatively late stage most of them soon disappear completely, and the 

 material in the two chromatic lumps is gradually distributed more or 

 less evenly along the parallel strands, which represent the daughter 

 chromosomes resulting from the split. 



The double chromosomes now shorten and thicken, forming the 

 "thick spireme" so conspicuous in prophase nuclei (Fig. 53, T, U). As 

 pointed out in the preliminary sketch of mitosis, the chromosomes in the 

 prophase may form a more or less continuous spireme, but it is becoming 

 increasingly apparent that this is not a universal phenomenon. It is 

 certain that in many cases the chromosomes are separate from the first, 

 and it seems therefore that any association in the form of a continuous 

 spireme is a matter of secondary importance. As the shortening and 

 thickening proceed the split may become obscured by the close 

 association of the halves, but suitable methods reveal its presence. 



While indications of spindle formation are appearing in the cytoplasm 

 the nucleolus disappears and the nucleus begins to contract, so that the 

 thick double chromosomes become very closely packed together. While 

 the contraction is at its height the nuclear membrane disappears, after 

 which the chromosomes loosen up as an irregularly arranged group. 

 This contraction stage evidently does not occur in many mitoses: the 

 membrane may disappear while the nucleus has its full size. However, 

 when it does occur it is of very short duration, so that it may take place 

 in more cases than has been supposed. After the disappearance of the 

 nuclear membrane the spindle fibers establish connection with the chro- 

 mosomes, which quickly become arranged with their halves in superposi- 

 tion at the equatorial plane, as described in the paragraph on the 

 metaphase. This brings us to the point with which our description 

 began. 



It should be added that in many descriptions of mitosis, notably 

 those presented in general text books, the chromosomes are said to split 

 during the metaphase, after they have become arranged upon the spindle. 

 Such a late development of the split may indeed occur in some cases, but 

 it is not improbable that closer examination would often reveal the 

 inception of the process at a much earlier stage. As has been pointed 

 out in the foregoing description, the early formed split frequently be- 

 comes obscured during the later prophases owing to the shortening and 

 thickening of the chromatin threads, and becomes conspicuous again 

 only after the metaphase figure has been established. 



Chromomeres. One matter which should receive special attention is 

 thatof the chromomeres. It was held by Roux (1883) that the compli- 



