THE SCOPE AND METHODS OF GENETICS 289 



to heredity; (/) theories as to the mechanism that brings about the 

 observed regularity in heredity, including theories of linkage, cross- 

 overs, and other phases of neo-Mendelian heredity. 



THE IMPORTANCE OF THE CELL THEORY IN GENETICS 



All organisms are composed of minute units called cells. A cell 

 is the smallest particle of living matter capable of growing and multi- 

 plying. The only continuity between two successive generations is a 

 cellular continuity, involving a continuous series of cell divisions. 

 All cells are the offspring by division of previous cells. We distinguish 

 two main categories of cells: body or somatic cells and germ cells. 



In germinal reproduction, the only kind of reproduction possible 

 in the more highly differentiated animals, the material continuity 

 between parent and offspring is through the germ cell. A parental 

 germ cell produces an offspring. The germ cell therefore is called the 

 hereditary bridge. 



The only way, then, in which a parent can transmit his or her 

 characteristics to an offspring is through the germ cell. Every 

 hereditary character, whether old or new, must have its differential 

 cause in the germ cell. The germ cells are therefore called the bearers 

 of the heritage, and in the next chapter Professor M. F. Guyer gives 

 an account of the cellular basis of variation and heredity. 



