44 - TheCel 



Fig. 3-3. Mitotic apparatuses, or spindle-aster-chromosome complexes, removed from dividing egg cells (sea 

 urchin) by the method of Mazia and Dan. A, late metaphase; B, prophase, at time of breakdown of nuclear 

 membrane. To isolate the apparatus one uses very cold ( — 12°C) 30 percent alcohol as a stabilizing agent; 

 then a detergent (digitonin, 1 percent) is employed to disperse the cytoplasm. (Courtesy of Daniel Mazia, Uni- 

 versity of California.) 



Moreover, as may be judged from the very 

 precise observations of Donald Costello, of 

 the University of North Carolina, the orien- 

 tation displayed by the paired centriolar rods 

 at one mitosis exerts a determining influence 

 upon the plane of division in the next 

 mitosis. 



The spindle likewise displays a duplex 

 structure. One set of fibers, collectively re- 

 ferred to as the central spindle, extends full 

 length from pole to pole. The other set. 

 which is usually designated as traction fibers, 

 originates from the poles, but each terminates 

 by making connection with the kinetochore 

 of some one of the chromosomes (Fig. 3-2). 

 When the spindle has reached full develop- 

 ment, the nuclear membrane usually has 

 begun to disappear (Fig. 3-2). However, in a 



few exceptional cases it has been observed 

 that kinetochore connections can be estab- 

 lished even while the membrane continues to 

 persist. In any event, there is one important 

 rule that is never broken, except in rare acci- 

 dental cases. The members of a sister pair of 

 kinetochores, formed as each chromosome 

 divides into two daughters, never establish 

 connection with the same pole of the spindle. 

 In other words, the two daughter chromo- 

 somes, formed by the division of each parent 

 chromosome, become connected to opposite 

 poles of the spindle. 



Metakinesis and Metaphase. After the dis- 

 appearance of the nuclear membrane, the 

 pairs of daughter chromosomes, each daugh- 

 ter lying parallel to and in contact with the 

 other, are arranged more or less randomly 



