Breeding 433 



vibrations of its tail-like appendage. Fertilization, or 

 the successful union of the egg and sperm, is thus condi- 

 tioned on the vigor and activity of the sperm. There being 

 in a normal breeding service many thousand sperms for 

 each egg produced by the sow, the sperms which are most 

 active and get to the eggs first are the ones which fertilize 

 them. Only one sperm ever unites with an egg under 

 normal conditions. 



As soon as the eggs are fertilized, they attach themselves 

 to the lining membrane of the uterus and begin to absorb 

 nourishment through its walls. Each individual egg 

 grows in size and divides into two ; each in turn absorbs 

 nourishment, grows in size, and divides into two, so that 

 where at first there was only the single fertilized egg-cell, 

 there are now four cells. This change, called cell divi- 

 sion, represents the beginning of embryonic development. 

 The process of cell division continues throughout the 

 gestation period and, under normal conditions, in about 

 113 days so-called birth takes place. 



"Like begets like.'' 



Within each cell resides the hereditary material which is 

 being passed on from parent to offspring. The extraordi- 

 nary fact is that although these germ-cells are so minute 

 as to be invisible, yet they contain every particle of heredi- 

 tary matter, every influence, which the parent contributes 

 to the unborn pig. The pig inherits nothing which he does 

 not receive in these two cells, one supplied by the sire and 

 one by the dam. 



When the male and female germ-cells unite to form the 

 fertilized egg, there is consequently a union of the heredi- 

 tary qualities contributed by the boar and sow. Since 

 each germ-cell carries a full set of characters, it follows that 



2F 



