46 THE PHYSIOLOGY OF REPRODUCTION 



berger/ however, gives from twenty to thirty days as the length 

 of this interval. The procestrum and oestrus together do not 

 as a rule occupy more than two or three days, and oestrus alone 

 may last for only a few hours. The external signs of the 

 procestrum are comparatively slight in sheep. The vulva is 

 usually somewhat congested, and there is often a flow of mucus 

 from the external generative aperture, but blood is seldom 

 seen. Owing to the extreme shortness of the " heat " period the 

 mucous flow may continue during the cestrous and metoestrous 

 periods. The internal changes are briefly described in the 

 sacceeding chapter. The only external indication of oestrus is 

 that afforded by the behaviour of the ewes. At this time they 

 tend to follow the ram, and display a general restlessness of 

 demeanour. The period of gestation is twenty-one or twenty- 

 two weeks. Nathusius' observations show that it is fairly 

 constant within the hmits of particular breeds.^ 



The cestrous cycle in the sheep, and its great variabiUty, 

 have been discussed at some length, since this animal is pro- 

 bably typical of most Ungulata in the way in which its generative 

 system is affected by different conditions of hfe, while the facts 

 about other Ungulates are not so perfectly known. The effect 

 of changed conditions upon the sheep's fertihty, i.e. upon its 

 capacity to bear young (as distinguished from mere sexual 

 capacity), is a subject which is dealt with more fully in a future 

 chapter (Chapter XIV.). 



The wild goat, Mke the wild sheep, has a very restricted 

 sexual season,^ while, according to Low, the domesticated goat 

 experiences oestrus at very frequent periods.* 



A similar statement may be made about cattle, for Heape ^ 

 says that, whereas wild cattle in captivity are capable of re- 

 production at any time of the year, and experience a remark- 

 able increase in the recurrence of their dioestrous cycles, we 

 are led to infer from the hmited calving season among 



1 EUenberger, VergUichende Physiologie der Haussaiigethiere, vol. ii. Berlin, 

 1892. 



" Nathusius, " Ueber einen auffallenden Racenunterschied in der Triichtig- 

 keitsdauer der Sohafe," Zool. Garten, Jahrg. 3, 1862. 



' Lydekker, loc, eit. 



" Low, Tlie Domesticated Animals, London, 1845. 



* Heape, loc. eit. 



