CELL MULTIPLICATION 87 



place simultaneously, or one of them may precede the other; 

 the relative order of these changes varies and is a matter of 

 no especial importance. 



2. Metaphase. The second stage in cell division is a very 

 important one and is really the key to the process. Each of 

 the chromosomes splits lengthwise into two identical halves, 

 which at first are parallel, as at D. This splitting of the chro- 

 mosome into identical halves is for the purpose of dividing 

 equally the chromatin material, so that the two cells which 

 are to arise from the original cell may each contain one-half 

 of the chromatin rods of the original cell. The fact that the 

 chromosomes split lengthwise is of significance, for it is mani- 

 fest that if the thread splits lengthwise, the two halves will 

 be essentially identical, while if it should divide crosswise, 

 the two halves would not be necessarily alike. In the equa- 

 torial plate, at E, these eight chromosomes become slightly 

 flattened and are drawn more closely together. 



3. Anaphase. In the third stage, the two halves of each 

 chromosome begin to move apart. As shown at F, four of 

 the chromosomes move away from the equatorial plate toward 

 each of the two centrosomes. There is little doubt that the 

 minute fibers which connect the poles of this spindle are con- 

 cerned in the separation of these chromatin threads, though 

 exactly how they work is not known. Finally, the separate 

 halves of the chromatin thread are brought close to the minute 

 granules lying at the two ends of the spindle, at G. 



4. Telophase. The last stage in the division simply com- 

 pletes the process, for the essential feature of division has 

 already occurred. The chromatin threads, which have come 

 to lie near the pole of the spindle, now combine and form a 

 network, at G, much like that present in the original nucleus, 

 and a nuclear membrane forms around this mass of chromatin 

 material at H. The minute granule within the center of the 

 spindle pole is divided in two, either now or later; and thus 

 a complete nucleus is produced with a centrosome beside it, 



