THE CELL THEORY 



257 



membrane, is another illustration of the complex structure 

 of the cell. Although the centrosome, as this spot is called, 

 has been heralded as a dynamic agent, there is not complete 

 agreement as to its purpose, but its presence makes it necess- 

 sary to include it in the definition of a cell. 



The Cell in Heredity. — The problems of inheritance, in 

 so far as they can be elucidated by structural studies, have 

 come to be recognized as problems of cellular life. But we 

 cannot understand what is implied by this conclusion without 

 referring to the behavior of the chromosomes during cell- 

 division. This is a very complex process, and varies some- 

 what in different tissues. We can, 

 however, with the help of Fig. 79, 

 describe what takes place in a typical 

 case. The nucleus does not divide 

 directly, but the chromosomes congre- 

 gate around the equator of a spindle 

 (D) formed from the achromatin; they 

 then undergo division lengthwise, and 

 migrate to the poles (E, F, G), after 

 which a partition wall is formed divid- 

 ing the cell. This manner of division 

 of the chromosomes secures an equable 

 partition of the protoplasm. In the 

 case of fertilized eggs, one-half of the 

 chromosomes are derived from the 

 sperm and one-half from the egg. 

 Each cell thus contains hereditary 

 substance derived from both mater- 

 nal and paternal nuclei. This is briefly the basis for re- 

 garding inheritance as a phenomenon of cell-life. 



A diagram of the cell as now understood (Tig. 80) will 

 be helpful in showing how much the conception of the cell 

 has changed since the time of Schleiden and Schwann. 

 17 



Fig. 80. — Diagram 

 of. a Cell. (Modified 

 after Wilson.) 



