PRINCIPLES OF SWINE BREEDING 25 



first well informed on the general principles of biology 

 that underlie reproduction. He cannot appreciate the 

 importance of heredity, the necessity of selection and the 

 value of variation until he does this. To understand 

 these principles he must first appreciate the cell and the 

 part it plays in the animal kingdom. He must look upon 

 the cell as the biological unit. He must know what cells 

 are, how they act, and why they are so important. He 

 must know something of cell structure, functions and 

 processes. 



The cell. The entire body of any animal is made up of 

 cells. These cells may partake of a variety of shapes and 

 functions. In the simplest form it consists of a mass of 

 protoplasm with a nucleus, with the whole inclosed in a 

 cell wall. Within this cell absorption, assimilation, 

 growth and reproduction take place. The reproductive 

 function is of greatest importance, since the process of 

 growth and production of the animal is dependent on 

 cell division. The process of cell division is quite com- 

 plicated. The somatic cells, or body cells, are being 

 built up and destroyed continuously, but the reproductive 

 or germinal cells live from the time the animal is con- 

 ceived until they are expelled, or until the animal dies. 

 Therefore, the breeder should look upon the animal as 

 made up of millions of units, and before the entire animal 

 can be understood the functions and process of the in- 

 dividual units must be understood and appreciated. 



The male germ cell (spermatozoon). This cell origi- 

 nates in the germinal cells in the testicle. After several 

 divisions, they are fully developed spermatozoa, and pass 

 from the testicle as mobile cells, consisting of a head, 

 which contains the nucleus, a neck, and mobile tail. This 



