30 THE FIRST PRINCIPLES OF HEREDITY 



(a) Karyokinesis. 



We have already explained that each cell possesses a 

 nucleus, which is the most important part of the cell. This 

 nucleus has a limiting membrane, and is filled with a net- 

 work of a substance called " Linin," while on this network is 



arranged the " Chroma- 

 tin," so called because it 

 is easily colourable with 

 artificial stains. On this 

 chromatin has been 

 centred the greatest in- 

 terest, because it is the 

 substance which has to 

 be looked upon as the 

 bearer of the hereditary 

 qualities of the cell, and 

 therefore of the organ- 

 ism. Apart from other 

 arguments, which will 

 appear later, this be- 

 comes evident from the 

 role the chromatin plays 

 in the division of the 

 cell. This division, 

 which goes by the 

 name of " Mitosis," 

 or " Karyokinesis," 

 takes place in a most 

 regular and exact manner, and serves the purpose 

 of dividing the chromatin substance into two equal 

 parts. 



Starting with the resting phase of the cell, before it 

 begins to divide, we see the arrangement of the nucleus 

 with its chromatin, as just described, and in it a little 

 body, the " Nucleolus," the function of which has not yet 

 been elucidated. Just outside the nucleus lies the small 



a 



Fig. 20. — A Cell. 



{From H. W. Conn, " Story of Life's 

 Mechanism.") 



protoplasmic network ; b, liquid in 

 its meshes ; c, nuclear membrane ; 

 d, nuclear network ; e, chromatin 

 net^vork ; /, nucleolus ; g, centro- 

 some ; h, aster; i, vacuole or air- 

 space ; /, inert bodies. 



