THE BIOLOGICAL BACKGROUND OF GENETICS 191 



lower organisms are unicellular, but all of the higher organisms are 

 multicellular. 



A complex organism such as a man or an insect consists of millions 

 of cells. Each cell bears a life of its own, but is dependent for its 

 supplies of food and oxygen upon other cells, and in turn performs 

 some function that is of value to the organism as a whole. The divi- 

 sion of labor and interdependence of the innumerable cells of the cell- 

 republic, the organism, is reflected in the fact that groups of cells of a 

 particular sort constitute tissues, that various combinations of tissues 

 form organs, and that various combinations of organs form systems, 

 such as the nervous system, the digestive system, the circulatory sys- 

 tem, and others. No matter how elaborate the specialization of or- 

 gans and systems may be, every living part is composed of cells. Cells 

 are perhaps the most fundamental units of life in about the same sense 

 that atoms are the fundamental units of matter itself. Also, cells 

 possess certain mechanisms which are the principal agents both in 

 preserving constancy of form and function (heredity) and in promoting 

 diversity and change (variation). Hence, if we desire to understand 

 how evolution takes place, it behooves us to make a careful examina- 

 tion of the constitution of cells. 



A TYPICAL CELL AND ITS COMPONENT PARTS 



Cells vary in size and in form according to the special functions 

 they subserve; but in spite of their numerous specializations they have 

 many features in common. The diagram of a cell shown in Figure 40 

 is not meant to represent any particular kind of cell, but is a composite 

 of many kinds of cells. Of course, no one kind of cell contains all of 

 the cell organs shown in this diagram. 



The two most important subdivisions of the cell are the nucleus 

 and the cytosome, or cytoplasm. 



The nucleus is a more or less centrally situated body, commonly 

 spherical in form, and separated most of the time from the cytosome 

 by a nuclear membrane. Within the nucleus is a characteristic material 

 known as chromatin, which at times condenses into definite bodies 

 known as chromosomes, each chromosome consisting of a bundle of 

 genes, or hereditary units. 



The cytosome consists of all the rest of the cell except the nucleus. 

 All the vast hordes of cells of different sorts constituting a given com- 

 plex organism are identical in their nuclei, but in any cell the cyto- 

 plasm differs in its form and structure according to its location in the 



