THE PERIODICITY OF (ESTRUS 147 



tions where pseudo-pregnancy should normally be produced, 

 namely, after sterile copulation. Experiments showed, however 

 that this did not occur in all cases (500). It seems probable 

 that the mechanism concerned in the stimulation of luteal 

 tissue after sterile copulation also affects the tissues of the 

 sterilized ovary. Some X-rayed mice, when mated, will 

 actually copulate every four or five days, namely, at the 

 periodicity which is characteristic of oestrus in the unmated 

 mouse. In the majority, however, this is not found, and 

 after one or two oestrous periods attended by copulation, the 

 oestrous cycle gradually fades out. 



That the Graafian follicle is not necessarily the essential 

 source of oestrin is emphasized by quantitative examination of 

 the liquor folliculi and of the residual tissue of the ovary after 

 removal of all large follicles (see p. 107). This examination has 

 shown that oestrin is fairly equally divided between the fol- 

 licles and the stroma (but see also 613), and it is obvious 

 that the stroma has at least as good a claim to be considered 

 the site of origin of oestrin as has the follicular tissue. The 

 X-ray work reveals the ovary in quite a new light, and two main 

 problems are raised: 



(a) If follicular maturation does not initiate oestrus, by what 



means can the synchronization be arranged ? 



(b) How is the periodicity of oestrus regulated ? 



Periodic action of oestrin. It is possible that oestrin is produced 

 at intervals and so achieves its periodic action. It must be re- 

 membered, however, that the oestrus-producing hormone can be 

 extracted from the ovary even when oestrus is completely in 

 abeyance, as during anoestrus and pregnancy. If, as seems 

 probable, the production of oestrin by the ovary is continuous, 

 some periodic mechanism, such as the cyclic attainment of a 

 threshold value, must exist. It has also been suggested that the 

 accessory organs have a periodic increase and decrease in 

 sensitivity to the oestrus-producing hormone. This supposition 

 was based on the idea that continuous injection of oestrin would 

 not prolong the oestrous changes indefinitely. More recent work, 

 however, has shown that cornification can be prolonged at will 

 and therefore that no periodic uterine sensitivity occurs (see 



