160 Richard M. Fraps 



to the second concept, follicles attain to maturity or ovulahility at fairly 

 regular intervals in cycles or sequences of given length. It is recognized that 

 follicular maturation is a gradual process, and ovulahility therefore a relative 

 condition. 



The interaction of these two "cycles" means essentially that if a follicle 

 attains to ovulahility at a sufficiently early time within the diurnally recurrent 

 period of elevated LH levels, it is ovulated. But if a follicle, i.e. the oncoming 

 Cj follicle, comes to high sensitivity too late in a given LH release period, 

 its ovulation is carried over to the following period, by which time it has 

 become highly responsive, presumably in part because of the maturation 

 promoting action of LH during the preceding night when this follicle 

 approached but did not quite achieve response to prevailing LH levels. With 

 some minor qualifications concerning follicular maturation, Bastian and 

 Zarrow believed their hypothesis capable of accounting also for lag in the 

 sequence, and thus for the "asynchronous ovulation rhythm of the 

 hen". 



Bastian and Zarrow advanced no direct evidence in support of their main 

 premise, the recurrent and prolonged secretion of LH during the same hours 

 of each 24. This concept appears to have had its origin in recognition of the 

 synchronization with photoperiod of the hours of the 24 within which OIH 

 release occurred, together with participation of the central nervous system in 

 control of pituitary gonadotropin secretion. If some phase of photoperiod, 

 such as onset of darkness, invariably activated the nervous component of 

 the system and thus caused the pituitary to release LH, such release would 

 of course be expected on the night preceding the day of no ovulation. As we 

 have seen earlier, release of LH during this night appears to be unnecessary 

 to account for the subsequent high terminal ovulahility of the C^ follicle, a 

 conclusion which of itself need not disprove the postulated release. But if 

 LH is actually so released, the oncoming Ci follicle would be subjected, 

 through at least a part of the night preceding LH release for its ovulation, 

 to what in effect would appear to be subovulatory levels of the gonadotropin, 

 a condition which often results in atresia, as Bastian and Zarrow (3) them- 

 selves and others (21, 22, 64) have found experimentally. Atresia might also 

 be expected to intervene at or near the termination of the ovulating sequence 

 if in fact the ovarian follicles mature at fairly regular intervals, as is postulated 

 by Bastian and Zarrow. The grounds for questioning the validity of this 

 second postulate have been discussed in connection with follicular maturation 

 and need not be repeated here. It is of some interest to observe, however, 

 that even though the order of follicular maturation described earlier and 

 represented in Fig. 2 be accepted, the release of LH during the night preceding 

 the release actually effecting ovulation of the Cj follicle — during the 

 period, p, of the figure — might also be expected to result not infrequently in 

 atresia. 



