260 THE BREEDING OF ANIMALS 



a large frog differs from a small frog in the number of 

 cells, not in their size. Each individual animal begins its 

 existence as a single cell. Through cell division the em- 

 bryonic organism rapidly increases in size until maturity 

 is reached. The rate of growth of the individual is most 

 rapid during the very early stages in the development of the 

 embryo. The rate of growth decreases gradually from this 

 time until full maturity, when nominally growth ceases. 



247. When the growth impulse is strongest. The 

 growth impulse is strongest in the animal while still exist- 

 ing in the uterus of the mother. After birth the growth 

 continues less rapidly, but is still very rapid when compared 

 with the increase in size during the later months of the 

 growth period. It is for this reason that the period of 

 gestation is so fundamentally important in the life of the 

 animal. During this period of exceedingly rapid increase 

 in size and development of the vital organs and other parts 

 of the body, any abnormal condition which interferes 

 with the normal requirements of the unborn animal may 

 cause arrested development and result in seriously retard- 

 ing the growth or permanently crippling the individual. 

 It is undoubtedly true that during this period the prac- 

 tical breeder may through skillful feeding and care ma- 

 terially influence for good or evil the development of the 

 valuable characters of the domestic animals. 



248. Development of the foetus. The development 

 of the fertilized egg through the embryonic stages of the 

 life of the mammalian animal is influenced by a number 

 of conditions which may have a profound influence upon 

 the material well-being of the future mature animal. 

 Some of these influences are as yet obscure and not well 

 understood, while others are more clearly determined 

 and their effects more easily recognized. 



