6 Farmers' Bulletin 1167. 



chromosome (which determines sex), they are always in pairs. 1 In 

 many of the domestic animals and in man the number of chromo- 

 somes is between 40 and 48, depending on the species. Just why this 

 number should be limited and invariable we do not know. It is no 

 more remarkable, however, that each cell in the hog's body, for ex- 

 ample, should have 40 chromosomes (which is the case) than that each 

 normal hog should have four legs. 



This is easily understood until we come to explain what happens 

 when the egg cell is fertilized at conception. Obviously an egg 

 with 40 chromosomes and a sperm cell with 40 chromosomes could 

 not unite, because that would result in a cell with 80 chromosomes and 

 only 40 is the rule. It just wouldn't work, any more than a pig 

 with eight legs would work. Therefore, to prevent the doubling of 

 the number of chromosomes at conception, nature puts the germ 

 cells, both male and female, through a maturing process known as 

 maturation, so that, just before the two unite, each has only half 

 the normal number of chromosomes for the species, one from each 

 of the pairs present before maturation. 2 When the two cells unite, 

 therefore, the fertilized egg has the usual number of chromosomes' 

 carried by the cells of the species. This process occurs only in the 

 case of reproductive cells. 



When two animals are mated, what happens? The reproductive 

 cells have gone through the maturing process and are now ready 

 for union. The sperm cells are active and move around until they 

 come into contact with the egg. Several may surround the egg, but 

 only one unites with it. The membrane surrounding the egg cell is 

 pierced by the head of a sperm cell. The two become one and the 

 egg is now fertile. It needs only nourishment to become in time a 

 full-fledged baby member of its race. 



Food, of course, will have a profound influence on this little 

 mite of life, but, so far as we know, the character of the resulting 

 animal, its sex, its identity, and its individuality, whether it is to 

 be white or black, long-haired or short-haired, ring-streaked or 

 spotted, are now settled by the laws of life. This minute cell now car- 

 ries in itself all the force of inheritance from all the ancestors behind 

 it to the beginning of life on earth. Every champion you or I have 

 known began his career in just this way. Many, many influences 

 stepped in and had an effect on his subsequent development. Some of 

 these are easy to determine, but others are so obscure that theories 

 and fancies have grown up to explain them. 



What value has this information for the practical breeder? Can 

 he use it in making his methods more efficient? He can not use it 



1 For a discussion of the accessory or X-Chromosome, the reader is referred to textbooks. 



2 Biologists call this process spermato genesis in the case of the male germ cells and 

 oogenesis in the case of the female germ cells. 



