Cell Division 



105 



spindle, however, we must assume some 

 chemical bonding between the micelles even 

 though these bonds are weak enough to be 

 easily broken and reformed, so that a chrom- 

 osome or a microdissection needle can move 

 through a chromosomal fiber without de- 



Fig. 20. Pollen mother cell of Lilium, centrifuged 

 (arrow indicates direction of centrifugal force) 

 (after Shimamura, '40). The chromosomal fibers 

 anchor the chromosomes to the spindle in the upper 

 pole. The chromosomes are uncoiled by the centri- 

 fugal force. 



stroying it permanently (cf. Ostergren, '49). 

 The centrifuge experiments of Shimamura 

 ('40) are impressive evidence for the strength 

 of the chromosomal fibers (Fig. 20). Still 

 too many questions remain completely un- 

 answered. What is the material forming these 

 fibers? What is its origin, and what is the 

 role of the kinetochore? What forces cause 

 their orientation to opposite spindle poles? 

 How are they anchored on the chromosome 

 and on the spindle? How do they function 

 as they move the chromosomes? At present 

 it seems most plausible to consider them as 

 a contractile, fibrous gel reversibly liquefied 

 with hydrostatic pressure (Pease, '46). 



Some investigators have considered chrom- 

 osomal fibers to be a liquid material se- 

 creted by the chromosomes and adsorbed on 

 the continuous fibers of the spindle. They 

 are thought to pull the chromosomes along 

 the spindle by means of surface forces 

 (Belaf and Huth, '33; Kupka and Seelich, 

 ''48). However, surface forces could hardly 

 keep the chromosomes attached to the spindle 

 with the centrifugal forces used (Beams 

 and King, '36) or against the pull of the 

 microdissection needle (Carlson, '52). 



After this discussion of the mitotic or- 

 ganelles we must now see how they effect 

 the congression of the chromosomes into the 



metaphase plate and the anaphase move- 

 ment to the spindle poles. 



Metakinesis. The chromosome movements 

 of metakinesis result in a regular arrange- 

 ment of chromosomes in the equatorial plane 

 of the spindle (metaphase plate). To accom- 

 plish this the following conditions must be 

 fulfilled: (1) a bipolar spindle must be 

 present; (2) the chromosomes (mitosis) or 

 chromosome pairs (meiosis) must be attached 

 to opposite spindle poles through chromo- 

 somal fibers. 



If the centrosome fails to divide in pro- 

 phase a single large aster may appear (mon- 

 aster). In Urechis, Belaf and Huth ('33) 

 found that chromosomes orient in the aster 

 and form chomosomal fibers toward and 

 away from the center. But no metaphase 

 plate is formed. In microspore mother cells 

 of certain hybrids or haploids in plants the 

 univalents usually do not form chromosomal 

 fibers and do not congress on the metaphase 

 plate. In Fig. 21^4 we see the spindle in a 

 pollen mother cell of haploid Datura. No 

 chromosomal fibers are present and the uni- 

 valents are scattered over the spindle. In the 

 diploid the bivalents produce oriented chro- 

 mosomal fibers and a metaphase plate is 

 present (Fig. 21B). Where chromosomal 

 fibers point to one pole only no metaphase 

 plate is formed, as in Sciara (Metz, '33) and 

 Micromalthvis (Scott, '36). In spermatocytes 

 of many insects (e.g., grasshopper) the uni- 

 valent X-chromosome is attached to one pole 

 only and does not go on the metaphase 





ti^ 



B 



Fig. 21. First meioti( divisidu in microspore 

 mother cell of Datura. A, Haploid Datura; continu- 

 ous fibers, but no chromosomal fibers are present; 

 no metaphase plate is formed. B, Diploid Datura, 

 chromosomal fibers present; chromosomes have 

 moved into the metaphase plate. (From slide of Dr. 

 Satina.) 



plate. In tctraploid spermatocytes of the 

 mantid, Callimantis, the X-chromosomes lie 

 on the metaphase plate if they are paired 

 and have chromosomal fibers to opposite 

 poles. Where pairing is absent, each uni- 

 valent has a single chromosomal fiber to one 



