THE ACHROMATIC FIGURE 163 



Meanwhile the polar regions of the spindle undergo little change. Long 

 chromosomes like those in Tradescantia undergo a marked shortening 

 during late anaphase. 



Although the normal spindle is frequently longer at the end of the 

 anaphase than in the metaphase, a number of workers ^^ object to the 

 emphasis on elongation as a major factor in mitosis. As urged by Bleier, 

 such an interpretation does not apply well to the mitotic phenomena in 

 many hybrids, where double chromosomes of different origin and con- 

 stitution (bivalents formed by synapsis and univalents which have split) 

 pass poleward at different times in the same spindle (Figs. 207, 208). 

 In Bleier's opinion, all hypotheses postulating only a passive movement 

 of the chromosomes, whether caused by streaming, a contraction of 

 fibers, or a "Stemmkorper," fail in such cases. It is rather to some force 

 resident in the chromosomes themselves that the movement is due 

 primarily; it is provisionally assumed that this is a repulsion force of 

 some kind. It is a suggestive fact that the chromosomes exhibit fully 

 their characteristic reactions in the spindle (attachment at its equator 

 and subsequent separation) only after they have reached a certain stage 

 in the development of their doubleness. The delayed attachment of the 

 univalents may be related to their late splitting, especially at their 

 attachment regions. The repelling force thus comes into play only when 

 the chromosome is properly constituted, and it is accordingly possible 

 to look upon the spindle as a structure which guides the chromosomes 

 instead of actively moving them. The existence of some such force is 

 further rendered plausible by certain other phenomena of synapsis and 

 disjunction to be considered in later chapters, as well as by the successful 

 distribution of daughter chromosomes in narrow root cells and pollen 

 tubes, where limitations of space often prevent the arrangement of the 

 chromosomes in a regular equatorial plane. 



Further evidence that the movement of the chromosome is not a 

 passive one is afforded by the remarkable monocentric mitosis discovered 

 in spermatocytes of flies of the genus Sciara by Metz.^* In the first 

 meiotic division, although all of the chromosomes show fibers extending 

 from their spindle-attachment regions toward the single pole, four of them 

 move backward away from the pole until they reach the cell boundary, 

 where they move into a compact group and are extruded in a bud from 

 the cell. The behavior of these chromosomes when their extrusion is 

 inhibited shows that they are not merely degenerated material of which 

 the cell is ridding itself. During their backward movement their shapes 

 suggest that a poleward force acting at the spindle-attachment region 

 is overcome by another force acting in the opposite direction. The 



1^ Martens (1929), Schaede (1929), Bleier (19306c). See also Schrader (1932). 

 18 See Metz (1925, 1926c, 1933) and Metz, Moses, and Hoppe (1926). See also 

 Huth's (1933) account of Belaf's work on monasters in Urechis eggs. 



