THE PERIODICITY OF (ESTRUS 135 



follicles and the appearance of cestrous symptoms in the acces- 

 sory organs. This correlation is as follows: 



{a) (Estrus and ovulation are synchronized throughout the 

 entire reproductive life of the animal. The first oestrus 

 appears with the first ovulation at puberty and the last 

 oestrus is synchronized with the last ovulation at the 

 menopause. The bat, as pointed out by Courrier (131), 

 is an exception to this rule. In this animal oestrus 

 occurs in the autumn and ovulation in the spring. 



{h} Where the breeding season is limited, the beginning and 

 end of the ovarian cycle are correlated wath the begin- 

 ning and end of uterine activity. 



(c) In the ferret and rabbit, where ovulation only occurs after 



copulation, mature follicles and the oestrous condition 

 persist together indefinitely in the absence of coitus. 



(d) During the luteal phase of the cycle, particularly during 



pseudo-pregnancy and pregnancy, when no follicles 

 normally mature, no symptoms of oestrus occur. 



(e) A condition of persistent oestrus in animals which nor- 



mally ovulate spontaneously is sometimes found in 

 conjunction with persistent cystic follicles in the 

 ovary. 



Since no uterine effect on the maturation of the follicles has 

 been demonstrated, this functional correlation throughout the 

 whole lifetime of the animal could most easily be explained on 

 the grounds that the oestrous cycle is regulated by the periodic 

 maturation of Graafian follicles. 



The tendency to emphasize the importance of the Graafian 

 follicle in the production of oestrus w^as accentuated when the 

 liquor folliculi w^as found to contain large amounts of the 

 oestrus-producing substance. Allen (11) claims to have shown 

 that the amount of the oestrus-producing substance which can 

 be obtained from liquor folliculi varies according to the stage of 

 maturation of the follicle. In this connection he remarks (8) 

 ' Its presence and absence, due to the periodic development of 

 successive sets of follicles, is sufficient to explain the mechanism 

 of oestrous phenomena. ' This author and his co-workers actually 

 maintain that the Graafian follicle elaborates the oestrus-produc- 



