THE PERIODICITY OF OESTRUS 137 



appearance of oestrus, although no subsequent recurrence is 

 found unless ovarian regeneration takes place. A similar 

 observation was made by Coward and Burn (146). The signi- 

 ficance of this discovery was not, however, appreciated im- 

 mediately. Brambell and Parkes (82) found that oestrus in the 

 mouse may occur up to 36-48 hours after double ovariectomy; 

 the only possible explanation of this phenomenon is that the 

 oestrus-producing stimulus becomes operative at about 48 hours 

 before its effect can be discerned by examination of the vaginal 

 smear. Histological examination of the ovaries removed from 

 mice coming into oestrus within the two days following the opera- 

 tion revealed the fact that the real maturation growth of the 

 follicle had not taken place. The average volume of Graafian 

 follicles not due to ovulate at the next oestrous period is about 

 three million /u^. This same size is maintained until about 

 halfway through the dioestrus preceding ovulation. When the 

 oestrus-producing stimulus becomes operative the follicles about 

 to ovulate have increased in size to an average of 3I million ju^. 

 During the two days preceding ovulation, enormous follicular 

 growth takes place, so that a follicle at the time of ovulation has 

 a volume of between eight and nine million /ul^. These facts 

 show adequately that in the mouse, at any rate, the maturation 

 of the follicle does not occur until after operation of the oestrus- 

 producing stimulus, and cannot, therefore, be responsible for 

 this stimulus. 



(c) OCCURRENCE OF CESTRUS AFTER FOLLICULAR 



ABLATION 



The real test of w^hether or not the maturation of the Graafian 

 follicle is the causative factor of oestrus is the effect of total 

 ablation of the follicles. The difficulty of completely destroying 

 the follicular system of the ovary has delayed such experimental 

 work. Marshall and Runciman (454) failed to inhibit the onset 

 of oestrus by rupturing the maturing follicles of the dog, and 

 hence considered that the presence of mature follicles was not 

 essential for the production of oestrus. Later, however, 

 Marshall and Wood (455) were unable to confirm these results. 

 Various doubtful points may be raised with regard to these 



