THE PHYSICAL BASIS OF HEREDITY. 



157 



to decide (Fig. 5, f}. Arrived at the poles the V- 

 shaped chromosomes become grouped in a star-shaped 

 figure, the " aster," their outer ends become again 

 joined together in the form of a tangled skein, the in- 

 dividual chromatin granules separate somewhat along 

 the threads of the linin network, their deeply staining 

 quality is decreased, and a new nuclear membrane devel- 

 ops around each group of chromosomes. Simultaneous- 

 ly with this the cytoplasm constricts across the middle 

 of a somewhat elongated cell, resulting in complete di- 

 vision in the equatorial plane of the spindle, and two 

 separate daughter cells result. Each of these is made 

 up of cytoplasm containing a centrosome and a nucleus, 

 similar in all respects to the parent cell from which it 

 has arisen. 



A simple tabulation of the changes just described is 

 as follows: 



Phases of Cell Division by Karyokinesis. 



( i. Resting nucleus. 

 I. Prophases j 2 Skein stage of chromatin . 



( 3. Segmented skein. 



II. Metaphase J 4- Equatorial plate and splitting of 



( chromosomes. 



S5. Movement of chromosomes to poles 

 and formation of 



6. Segmented daughter skeins. 



IV. Telophases. ... J 7- Reconstruction of nucleus. 



( 8. Division of cytoplasm. 



It is readily seen that the culmination of the process 

 lies in the splitting of the chromosomes and the separa- 

 tion of their component halves to form the two new 

 daughter nuclei. 



In the foregoing description of the changes typ- 

 ically passed through by an animal cell in division, 



