256 EVOLUTION AND ANIMAL LIFE 



of the greatest theoretical significance. By it the chromatin substance 

 of the original nucleus is equally distributed between the two daughter 

 nuclei, so that each receives a half of each original chromosome. The 

 elaborate mechanism and consequent expenditure of energy involved 

 in this careful longitudinal division of each chromosome, rather than 

 a simple mass division, such as might be brought about by far less com- 

 plicated means, indicates clearly that the distribution of the definite 

 organization of the chromatin to the daughter cells is of primary 

 importance, a conclusion w^hich is further strengthened by much 

 evidence too extended to be entered upon here. 



"In the 'anaphases' and 'telophases,' which include the closing 

 stages of division, the daughter chromosomes migrate along the fibers 

 of the central spindle toward its poles, perhaps through the direct 

 contraction of the mantle fibers under the influence of the centro- 

 some, though this and many other points regarding the forces at work 

 must be left for future investigation to decide. Arrived at the poles, 

 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 indi\ddual chromatin granules separate somewhat 

 along the threads of the linin network, their deeply staining quality 

 is decreased, and a new nuclear membrane develops around each 

 group of chromosomes. Simultaneously with this the cytoplasm 

 constricts across the middle of a somewhat elongated cell, resulting 

 in complete division 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 w^hich it has arisen. 



"A simple tabulation of the changes just described is as follows: 



Phases of Cell Divisio>f by Karyokinesis 



C 1. Resting nucleus. 



I. Prophases -j 2. Skein stage of chromatin. 



(.3. Segmented skein. 



.py AT f V, i '^' Equatorial plate and splitting of 



A.I., ivieuapnase j , 



( chromosomes. 



C 5. IMovement of chromosomes to poles 



III. Anaphases S and formation of 



V G. Segmented daughter skeins. 



TT,. ™ , , (7. Reconstruction of nucleus. 



IV. lelophases i o -ta- • • ^ . i 



( 8, Division of cytoplasm. 



