780 



REPRODUCTION AND MIGRATION IN BIRDS 



nervous activation of the pituitary fails to take place at about the time 

 Q or, indeed, at any time during the hours delimited as q. 



Mechanisms 



An hypothesis proposed earlier (Fraps, 1954, 1955a,b) is based on 

 the following assumptions: (i) The nervous component of the OIH 

 release mechanism exhibits a 24-hr periodicity in thresholds of re- 

 sponse to ovarian hormones; thresholds of response vary quantita- 

 tively during those hours of the twenty-four over which response may 

 occur; during the hours of lapse, thresholds are beyond the level of 

 response, (ii) Blood concentrations of ovarian hormones "exciting" 

 the neural component of the OIH release apparatus increase by sub- 

 stantially the same course after each release of OIH (or LH) in cycles 

 of given length. It follows that each "curve" of increasing excitation 

 hormone concentration beyond that associated with the first excitation 

 of a sequence is retarded, in time of day, by the extent of lag at the 

 preceding OIH release. 



Relationships between diurnally recurrent rhythmicity in the nerv- 

 ous component of the OIH release mechanism and excitation hormone 

 concentrations are shown schematically in Fig. 3 for a 7-day cycle {n 



Fig. 3. Relationships between diurnal rhythmicity in thresholds of 

 response by the nervous component of the OIH release mechanism and 

 excitation hormone concentrations for a 7-day cycle {n — 6). Zero hour, 

 about 10 P.M.; hour 8, about 6:00 a.m. 



= 6). The curve passing through El, Eo, E3 • • • Eg; E/ represents 

 the course of diurnal variation in thresholds of response to excitation 

 hormone (ordinates). This rhythmicity in thresholds of response is 

 assumed to recur daily. Curves Ci, C2, C3 • • • Cg and Ci' repre- 



