CHAPTER XVIII 



GENERATION AND DEVELOPMENT 



The Sexual Season of female animals is a subject which in recent 

 years has received exact expression at the hands of Heape,* whose 

 communication, quoted below, we have followed in connection 

 with this question. Heape divides female mammals into two 

 classes — Moncestrous, or those which have one cestrous cycle, 

 and Poly cestrous, or those having a series of oestrous cycles. The 

 first phase of generative activity at the beginning of a sexual 

 season is known as Procestrum, or the pro oestrous period ; it corre- 

 sponds to the period ' coming on heat,' or ' coming in season.' 

 The period lasts a variable time in different animals, and is 

 succeeded by the period of desire, or (Estrus. It is only during 

 this period that sexual intercourse is permitted, or that fruitful 

 coition is possible. If conception does not occur or is prevented, 

 oestrus is followed by Metcestrum, or the metcestrous period, during 

 which sexual activity passes away, and is succeeded by a period 

 of complete rest or freedom from sexual excitement, known as 

 Ancestrum. The ancestrous period may last two, three, eleven, 

 or more months, depending on the species. 



The sexual cycle is not always as above described ; there are 

 animals in which metcestrum is not followed by a period of 

 complete rest, but by a short quiescence, known as Dicestrum, 

 which lasts a certain number of days, and is then followed by 

 a new procestrum, oestrus, metcestrum, and dicestrum. Among 

 moncestrous mammals is the wolf, which in the wild state has 

 only one sexual season in the year. Another is the dog, though 

 in this case the sexual season may recur during the year ; but 

 the periods in each case are quite distinct, and followed by 

 complete rest, which is the essentially distinguishing feature. 

 Among polycestrous mammals are the mare, cow, sheep, pig, 

 and all of these during a portion of the year exhibit a series of 

 dicestrous cycles (in the absence of pregnancy), followed by 



* ' The Sexual Season of Mammals,' etc., by W. Heape, M. A., Quarterly 

 Journal of Microscopical Science, vol. xliv., p. i, 1901. 



688 



