34 THE BREEDING OF ANIMALS 



32. The nature of fertilization. The essential na- 

 ture of fertilization still remains a matter of discussion. 

 It may be that the fertilization of the egg is primarily 

 a rejuvenescence of the protoplasmic material which, for 

 some at present unknown reason, has lost the power of 

 further growth through cell division. This view was 

 held by Butschli, Hertwig, Minot, Engelman and others. 

 Even in the simpler forms of life where anything like 

 sexual union is absent, the cycle of growth is continually 

 reinaugurated by the conjugation of independent cells. 

 In all higher forms the egg is stimulated to growth and 

 cell division by the introduction of the sperm. 



Weismann has looked upon fertilization as a source 

 of variation and has maintained that this should be 

 regarded as the chief function of this process. Both 

 the theory of rejuvenescence and that which regards 

 this process as chiefly a source of variation are in accord- 

 ance with the observations of practical breeders. Cross- 

 breeding is known to induce greater vigor and increased 

 fecundity and at the same time to break up the fixed 

 characters of the breed or type. It is equally well estab- 

 lished that under certain conditions in-breeding tends to 

 identity of character and results in sterility and weakness 

 of constitution. But, after all, the real purpose and na- 

 ture of sexual union in reproduction is still an unsolved 

 problem. 



33. The process of fertilization. The ultimate pur- 

 pose of the sexual union of animals is to insure the fertili- 

 zation of the egg by the spermatozoon. When the egg 

 and sperm meet within the generative organs of the fe- 

 male, significant and important changes are set in motion, 

 which eventually result in an admixture or union of the 

 germ substance of the two parents. These changes are 



