180 PHYSIOLOGY OF FARM ANIMALS [CH. 



pigment. After three or four heat periods this jDignient maj^ have 

 accumulated in such quantity that ahnost the whole surface of 

 the uterine mucosa is rendered jet black, the pigment only 

 disappearing during subsequent pregnancy or during the an- 

 oestrum when it is said to be removed by leucocytes. 



Oestrus in the ewe may last for a day or for only a few hours, 

 and the prooestrum and oestrus together do not occupy more than 

 two or three days. Ovulation takes place during oestrus and the 

 number of follicles which discharge is usually one or two, some- 

 times three, but rarely more than three. 



The wild sheep (such as the Mouflon or the Barbary sheep) is 

 said to be monoestrous and to experience only one sexual season 

 annually. The Scottish Black-faced sheep in the Highlands is 

 generally dioestrous, that is to say, it can have two 'heat' periods 

 in the absence of the ram but not more than two, and the season 

 for tupping is November. In the Lowlands, however, where the 

 climate is less severe and food more abundant, there may be five 

 or six dioestrous cycles if the ewe fails to become pregnant before 

 anoestrum or prolonged quiescence is resumed. Amongst the 

 English breeds the number of recurrent 'heat' periods is generally 

 much greater and in Dorset Horns and Hampshire Downs the 

 ewes may exiDcrience oestrus at almost any time of the year. The 

 same is true of the Merino sheep amid favourable conditions, and 

 in New South Wales the sexual season is said to ejnbrace the whole 

 year, there being in the absence of pregnancy an almost unin- 

 terrupted succession of dioestrous cycles. Amongst English 

 varieties the Dorset Horns are almost alone in being able to have 

 two crops of lambs in one year, but this practice is discouraged as 

 it tends to cause too great a strain and thus deteriorate the ewe. 

 The capacity to experience oestrus or the degree of polyoestrum is 

 partl}^ a racial characteristic, but it depends at least equally upon 

 environment and nutrition which favour an increased fecundity. 



The period of gestation in the sheep is 2 1 to 22 weeks or about 

 five months, so that if lambing occurs twice in one year there is 

 only about a month's interval between the end of one pregnancy 

 and the beginning of the next. 



The cor]ous luteum, which as we have seen is formed from the 

 discharged follicle, develops very rapidly in the sheep. Its fate 

 dejjends upon whether or not gestation supervenes as a result of 

 coitus, for in the pregnant anitnal the corpus luteum goes on in- 



