THE ACHROMATIC FIGURE 161 



All such hypotheses regarding the mechanism of mitosis must be recon- 

 sidered in the light of what has recently been learned about the achromatic 

 figure in living cells. As stated above, the aster has been found to be a 

 semisolid region of the cytoplasm with centripetal fluid paths, but, since 

 many achromatic figures have no asters, the causes of mitotic chromosome 

 movement obviously lie primarily in the spindle and the chromosomes. 



It is a notable fact that in uninjured living cells the metaphase spindle 

 appears to be optically homogeneous, no "fibers" being visible. ^^ Its 

 density is such that it can be moved bodily through the cytoplasm with 

 the micromanipulator (Chambers) or the centrifuge, ^^ but the micro- 

 needles reveal no fibrous structure. In tissue cultures it is observed that 

 when the medium is made acid the fibers appear and the mitotic process 

 ceases; when it is made neutral again the fibers disappear and the process 

 is resumed (Lewis). The fibers can be made to appear by dehydration 

 and by various agents which bring about gelation or coagulation. Hence 

 the coarsely fibrous appearance of the spindle in many fixed preparations 

 is an unnatural one. 



It is not to be concluded from these facts that the striations and trac- 

 tile fibers observed in such preparations have no structural basis what- 

 ever in the living cell. We are now forced to view the untreated spindle 

 as a body with a longitudinally oriented finer structure in spite of its 

 optical homogeneity.^^ In support of this view Belaf cites the following 

 facts: the distal portions of long chromosomes in the spindle tend to 

 become arranged parallel to its axis; the Brownian movement of occasional 

 included granules is greater in amplitude lengthwise of the spindle than in 

 other directions; mitochondria which sometimes enter an artificially 

 swollen spindle tend to be arranged longitudinally; the spindle has a 

 pronounced axial resistance to contraction; certain agencies cause the 

 spindle to split longitudinally (Fig. 90, c). Hence the spindle may be 

 likened to a crystal w^hich appears homogeneous to the eye but can be 

 shown to have an oriented structure by suitable treatment, or to a block 

 of wood whose splitting and differential swelling indicate the presence of 

 parallel elements. 



The exact nature of the invisible local modifications which appear 

 upon fixation in definitely localized positions (notably near the spindle- 

 attachment regions of the chromosomes) is not yet known. The fixed 

 images often suggest that they are streams moving in harmony with the 

 cytoplasmic streaming frequently observed during these stages (Fig. 97) ; 

 but Belaf has shown that the movement of the chromosomes may con- 



1* Chambers (1914, 1915, 1917, 1924), Chambers and Sands (1923), Sands (1923), 

 M. R. Lewis (1923), Lewis and Lewis (1924), Martens (1927c, 1929), Robyns (1929), 

 Belaf (1927, 1929a6). 



i*F. M. Andrews (1915), Nemec (1927). 



1" Robyns (1926, 1929), Belaf (1927, 1929afc), Martens (1929), Nemec (1927, 

 1929e), Bleier (19306), Jungers (1931). 



