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 behavier 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. 7o, 
describe what takes place in a typical 
case. The nucleus does not divide 
directly, but the chromosomes congre- 
gate around the cquator of a spindle 
(D) formed from the achromatin; they 
then undergo division lengthwise, and 
migrate to the poles (H, 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 clerived from the 
sperm and one-half from the egg. Fic. 80.—Diagram 
Each cell thus contains hereditary of a Cell. (Modified 
‘ after Wilson.) 
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 (Fig. 80) will 
be helpful in showing how much the conception of the cell 
has changed since the time of Schleiden and Schwann. 
17 
