Cell Division 



107 



taneously (Fig. 225) (Ris, '49; Hughes and 

 Swann, '48; Hughes and Preston, '49). In 

 this case, it is possible to inhibit spindle 

 stretching without affecting the chromo- 

 somal fibers, demonstrating the relative in- 

 dependence of the two components also 

 where they act simultaneously (Ris, '49). 



In some cells spindle stretching may be 

 absent (the common situation in somatic 



zonal" fibers. Little is known about their 

 origin, but it is possible that their appear- 

 ance is related to the shedding of ribonu- 

 cleoprotein from anaphase chromosomes that 

 was mentioned above (Ris and Kleinfeld, 

 '52). 



The future study of anaphase movement 

 will have to be directed mainly toward an 

 experimental analysis of the mechanisms of 



Table 3. Maximum Rates of Chromosome Movement and Spindle Elongation during 



Anaphase 



TEMPERATURE, 



°c. 



chromosome movement* 

 micra/minute 



spindle elongation 

 micra/minute 



Hughes and 

 Swann, '48 



Due to contraction of chromosomal fibers only, 

 t Chromosomal fibers do not contract here. 

 X Combined rate not analyzed into contribution of chromosomal fibers and spindle elongation. 



cells of plants) ; in others the chromosomal 

 fibers do not contract and anaphase move- 

 ment is due to spindle stretching alone 

 (Ris, '43). Usually the chromosomes all 

 move simultaneously, but cases of autono- 

 mous movements of chromosomes are known. 

 This independent behavior of chromosomes 

 is based on the autonomy of the kinetochore 

 and chromosomal fibers. 



As the chromosomes approach the poles 

 the region of the spindle in between may re- 

 main a semisolid structure (for instance, 

 grasshopper spermatocytes) or it may solate 

 and disappear. This is demonstrated by a 

 decrease in viscosity (Carlson, '46), by the 

 penetration of cytoplasmic granviles into this 

 space (Ris, '43), and by the disappearance 

 of birefringence (Swann, '51). Chromosomes 

 at anaphase are often connected by "inter- 



chromosomal fiber contraction and spindle 

 elongation. The presence in the spindle of 

 phosphatases that split ATP (Biesele, '49) 

 suggests a possible role of high energy phos- 

 phate bonds and a certain similarity to the 

 contractile processes in muscle and myosin 

 gels (cf. Brachet, '50; Hayashi, '52). 



The velocity of chromosome movement as 

 determined in living cells is largest in the 

 early part and gradually decreases. The max- 

 imum rates of chromosome movement and 

 spindle elongation in a number of cells are 

 given in Table 3. 



CYTOKINESIS 



Following the separation of the chromo- 

 some halves the cytoplasm subdivides to 

 complete mitosis (for a general review see 



