THE MAMMALIAN SEXUAL CYCLE 45 



pregnancy and pseuclo-pregnancy and active secretion generally 

 occurs at or near the end of each of these periods, but the growth 

 of the glands and their subsequent activity are less complete 

 when true gestation does not take place.^ 



The gestation period in the dog is about sixty-two days, 

 and pseudo-pregnancy may be said to last for approximately the 

 same time or probably rather less. It does not terminate so 

 abruptly as true pregnancy. After either period the uterus 

 passes back to a condition of anoestrous quiescence. The occur- 

 rence of suckling which follows pregnancy favours the involution 

 of the uterus, which in true pregnancy attains an enormous 

 size, due largely to great muscular development. The changes 

 which take place in pseudo-pregnancy are to be regarded, to 

 speak teleologically, as intended to provide for foetal nutrition 

 and development, but do not actually serve this end owing to 

 the ova not having been fertilised. As w^ill be made clearer 

 later on, both series of changes are due to a stimulus coming 

 from the corpus luteum, which develops irrespectively of whether 

 the eggs are fertilised or not. 



Polywstrous animals differ from monoestrous ones in having 

 tw^o or more short recurrent cycles within one sexual season. 

 Thus in the sheep during " tupping time," the ewe may experience 

 a succession of cycles, each lasting about fifteen days, until 

 pregnancy is attained or the sexual season is over. Similarly, 

 the mare, the cow, and the sow each experience recurrent cycles 

 of three weeks. In all these animals the " heat " period itself 

 is of comparatively short duration, and the actual oestrus may be 

 reduced to a few hours as in the sheep or sometimes in the cow. 

 The prooestrous changes are relatively slight, and bleeding from 

 the uterine cavity is rare. Ovulation takes place during oestrus 

 (or very shortly after, as may happen in the cow). There is no 

 prolonged pseudo-pregnancy as in the monoestrous bitch, but the 

 corpus luteum (which is formed as usual from the discharged 

 folHcle) proceeds with its development for from one to two weeks, 

 and then undergoes involution, so that by the time a new 

 period is due this structure is in an advanced state of retrogression, 

 though still prominent on the surface of the ovary, and easily 



^ A secretion of a fluid resembling milk, but generally much thicker 

 in the later stages, is in some animals (cow, goat) secreted during 

 pregnancy (Hammond and Woodman, Asdell). 



