DEVELOPMENT 261 



The development of the foetus is influenced by heredity, 

 and the physiological environment of the pregnant mother. 

 Among the latter are the general health or well-being 

 of the mother, age, the quantity and quality of the food 

 supply and mental impressions. 



249. Heredity and foetal development. The inherited 

 tendencies of an animal are exhibited from the very be- 

 ginnings of its existence in the fertilized egg-cell. Its foetal 

 development, therefore, must be influenced to a certain 

 extent by those inherent determiners which have come to 

 the fertilized egg-cell from the male parent. The size of 

 the foetus at various stages of development then would be 

 determined, not alone by the maternal heredity and en- 

 vironment, but also by the inherited characteristics which 

 have been acquired through the male. At the same time 

 it is probable that the maternal environment has a much 

 more important influence on the foetal development than 

 its paternal hereditary tendencies. If it were otherwise, 

 serious consequences might follow the mating of smaller 

 females to much larger males among the domestic animals. 



Pony mares weighing 700 to 900 pounds are not infre- 

 quently mated with large stallions of the draft breeds 

 weighing 2000 pounds or more. Under these conditions 

 the foetus is not as much larger than the normal foetus 

 of the mother as might be expected from the much greater 

 size of the stallion. Small burro mares weighing 400 

 or 500 pounds have been artificially inseminated with 

 the sperm of Percheron and other heavy draft stallions. 

 The growth of the foetus in this case is undoubtedly some- 

 what greater during the normal period of gestation than 

 the growth of a pure burro foetus, but the increased size 

 is not a mean between the normal size of a Percheron and 

 a burro foetus, but is much smaller. 



