CELL DIVISION AND GAMETOGENESIS 223 



equivalent in every respect. The appearance of the mitotic 

 figure at this point suggests that each half chromosome is being 

 drawn toward a pole of the spindle, but nothing final is known 

 as to the mechanics of the process. In favorable material, 

 during the anaphase, each chromosome seems to be attached to a 

 fiber extending from the chromosome to the pole of the spindle 

 and known as a chromosome fiber or a half-spindle fiber. At 

 this same stage, as the ends of the chromosome separate, one or 

 two fibers stretch between the separated ends. These fibers are 

 called the interzonal fibers. There is some evidence to show that 

 chromosomal and interzonal fibers are formed by the chromo- 

 somes after they reach the spindle. The central or axial part of 

 the spindle seems to be formed as a result of activity in the 

 centrosomes. In endeavoring to understand the mechanics of 

 mitosis it is necessary to remember that the separation of the 

 chromosome into two halves is begun, in some cases at least, 

 before the chromosome reaches the spindle. Thus there may be 

 a repelling force operating to separate the chromosome halves, 

 begun in the nucleus and continuing on the spindle. Some 

 students of mitosis believe that both a "pulling" and a "push- 

 ing" factor are concerned in the process. 



The anaphase includes stages following the metaphase up to the 

 point where each of the two daughter groups of chromosomes 

 forms a dense mass about each pole of the spindle. When this 

 point is reached, mitosis is in the telophase or final phase of 

 division. In the late anaphase or early telophase, the centriole 

 at each pole of the spindle divides in two. A constriction in the 

 cytoplasm in a plane at right angles to the spindle begins to 

 appear at this time and gradually cuts the cell in two. The 

 spindle fibers and astral radiations then fade and a typical 

 nucleus forms in the region of the telophase chromosomes. The 

 chromosomes become diffuse and a nucleolus reappears. 



The most impressive feature of mitosis is the exact lengthwise 

 division of each individual chromosome into two equal parts, 

 resulting in each daughter cell receiving not only an equivalent 

 quantity of chromosome material but also an equivalent quality, 

 since each chromosome possesses qualities or characteristics that 

 distinguish it from other chromosomes. It is true, of course, that 

 the cytoplasm is also divided in equal amounts in mitosis, but in 

 this case only a mass division takes place. The achromatic 



