CHAPTEE II 

 THE CESTROUS CYCLE IN THE MAMMALIA 



" Omne adeo genus in terris hominumque ferarumque 

 Et genus sequoreum, pecudes, pictseque voluores 

 In furias ignemque ruunt : amor omnibus idem.'' 



— Virgil, Georg. iii. 



In 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,^ and afterwards adopted by me,^ 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 ruttiiig season; but in many species the 

 male animals are capable of copulating at any time, whereas in the 

 females this function is restricted to definite periods. 



The non-breeding season or period of rest in a female mammal, 

 when the generative organs are quiescent (at least relatively) and 

 the uterus is normal and comparatively anaemic, and the animal 

 shows no disposition to seek out a mate, is called by Heape the 

 Anjjestrous period or simply the Amestrum. This period is generally 

 considerably prolonged, and in many Mammals occupies the greater 



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



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

 in the Sheep," Phil. Trans., B., vol. cxcvi., 1903. " The CEstrous Cycle in the 

 Common Ferret," Qtum: Jom: Micr. Science, vol. xlviii., 1904. See also Marshall 

 and Jolly, " Contributions to the Physiolbgy of Mammalian Eeproduction : 

 Part I. The CEstrous Cycle in the Dog," Phil. Trans.„'Q., vol. cxcviii., 1905. 



