MAMMALIAN REPRODUCTIVE CYCLE 

 <- — 24hr.— > 



527 



DAY NIGHT 







Fig. 8.10. Deinoni^lration of 24-lioiir penodu-ity in the luteinizing hormone-release appa- 

 ratus of female rats (Vanderbilt strain, 4-day cycle, controlled lighting: 14 hours per day). 

 Schematic representations of the normal cycle (A) and of characteristic results of different 

 regimes of Nembutal treatment (B to F). Vaginal stages indicated by Roman numerals over 

 each time scale; symbols above these show the corresponding follicle and corpus luteum 

 stages. The device marked <S defines the "critical period," the time limits of pituitary activa- 

 tion as experimentally determined (Fig. 8.11). OV indicates normal ovulation iinic in .1 and 

 estimated ovulation time elsewhere. NBTL indicates intraperitoneal injection of Nembutal. 

 (From J. W. Everett and C. H. Saw.yer, Endocrinology, 47, 200, 1950.) 



large follicles, providing that on the second 

 and third days the dose is increased or 

 supplemented by a second injection. Omis- 

 sion of any of these injections results in 



ovulation during the ensuing night. Thus, 

 there is a clearly defined 24-hour rhythm 

 in the LH-release mechanism. The results 

 confirm a similar conclusion based on the 



