CHAPTEK II 



THE (ESTROUS CYCLE IN THE MAMMALIA 



" Omne adeo genus in terris hominumque ferarumque 

 Et genus aequoreum, pecudes, picteeque volucres 

 In furias ignemque ruunt: amor omnibus idem." 



ViRGlL, Georg. iii. 



IK describing the sexual processes of the Mammalia, and the 

 variations in the periodicity of breeding which occur in the 

 different groups, I have employed the terminology originally 

 proposed by Heape, 1 and afterwards adopted by me, 2 in giving 

 an account of these phenomena in the sheep and other animals. 

 The terms used may now be defined. 



The term sexual season is used by Heape to designate the 

 particular time or times of the year at which the sexual organs 

 exhibit a special activity. It is, in fact, employed in practically 

 the same sense as that in which the expression " breeding 

 season " is used in the previous chapter. Heape suggests that 

 it is better to adopt the latter term to denote " the whole of 

 that consecutive period during which any male or female 

 mammal is concerned in the production of the young/' since 

 the expression is often used to include the period of pregnancy 

 or even the period of lactation. The sexual season is the season 

 during which copulation takes place, but this only occurs 

 at certain still more restricted times, the periods of " oestrus " 

 (defined below). The male sexual season, when there is one, is 

 called the rutting season ; but in many species the male animals 



1 Heape, "The Sexual Season," Quar. Jour. Micr. Science, vol. xliv., 1900. 



2 Marshall, " The (Estrous Cycle and the Formation of the Corpus Luteum 

 in the Sheep," Phil. Trans. ., vol. cxcvi., 1903. " The (Estrous Cycle in the 

 Common Ferret," Qiiar. Jour. Micr. Science, vol. xlviii., 1904. See also 

 Marshall and Jolly, " Contributions to the Physiology of Mammalian Repro- 

 duction : Part I. The (Estrous Cycle in the Dog," Phil. Trans. B., vol. cxcviii., 

 1905. 



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