GENERATION AND INHERITANCE 



297 



DIAGRAM II. Six stages of cell-division and nuclear division (Karyokinesis). 



FIG. 1. The first stage. Cell in the resting spherical form, showing a nucleus and 

 one centrosome (c). The nucleus shows a network of linine with threads and 

 granules of chromatin (ch). 



FIG. 2. Second stage. During the preparation for division (pro phase) the 

 chromatin has drawn together into a thread which has immediately broken up 

 into four pieces (chromosomes). The centrosome (c) of Fig. 1 has divided, and 

 between the two parts a spindle has arisen. 



FIG. 3. Third stage. The spherical nucleus has dissolved. The two centrosomes 

 of Fig. 2 are more widely separated and the spindle between them has become 

 much larger. The four chromosomes (ch of Fig. 2) have arranged themselves 

 symmetrically in the middle of the spindle to form the mother star. 



FIG. 4. Fourth stage. The four chromosomes of the spindle have split longitud- 

 inally each into two daughter chromosomes (ch 1 and ch 2). 



FIG. 5. Fifth stage. The daughter chromosomes which arose by longitudinal 

 splitting have separated further and further from one another toward the opposite 

 ends of the lengthening spindle (formation of two daughter stars). The cell begins 

 to segment in the middle. 



FIG. 6. Sixth stage. The segmentation has become complete, and the mother cell 

 is thereby divided in half. In each daughter cell a spherical daughter nucleus, 

 which contains the chromatic substance of four daughter chromosomes (ch), has 

 arisen from half of the spindle. By each daughter nucleus (k) lies a centrosome (c). 



