54 THE BREEDING OF ANIMALS 



at lambing time as a result of this practice. It is undoubt- 

 edly true that the practice stimulates sexual activity 

 and does cause the ewes to come in heat with greater 

 regularity. It is probable that this result is due both 

 to the character of the feed and its abundance. 



The breeding season may be influenced by heredity. 

 Certain breeds of sheep will breed readily at all seasons. 

 The Dorset Horned breed comes in heat and breeds at 

 all seasons, while most of the mutton and merino sheep 

 breed readily only in the fall. Other conditions which 

 are known to influence the breeding season in domestic 

 animals are the kind of food, condition of the animal, 

 age and breed. 



Evvard * has shown that sows gaining in weight when 

 bred produced larger litters than sows that were fed only 

 a maintenance ration. 



51. Phases of the breeding season. The breeding 

 season is divided into more or less distinct phases and 

 these have been described and named by Heape 2 and 

 Marshall as the procestrum, oestrum, metoestrum and 

 dioestrum. 



52. Prooestrum. The first part of the sexual season 

 is occupied by the procestrum. This period is character- 

 ized by marked changes in the generative organs, the 

 uterus becoming congested, while in the later stages 

 there is often a flow of blood from the external opening 

 of the vagina. The procestrum is the period often re- 

 ferred to by breeders as the time when an animal is " com- 

 ing in heat " or coming in season. 3 



1 Evvard, in "Report of American Breeders' Association," 

 191. 



2 Heape, Quarterly Journal of Microscopical Science, vol. 44, 

 p. 1. 



3 Marshall, "The Physiology of Reproduction," p. 36. 



