664 PHOTOPERIODISM IN VERTEBRATES 



cient for commercial uses) by a shortening of day lengths alone, and 

 delayed by postponing the normal autumn shortening of day lengths. 

 The earlier, alternative procedure advanced maturity four months, 

 however, suggesting that the prior lengthening (or perhaps days over a 

 certain length) may hasten completion of a preceding phase of matura- 

 tion and pave the way for rapid final maturation when day lengths are 

 shortened, and perhaps an inhibition is removed. The effectiveness of 

 abrupt lengthening followed by abrupt shortening remains to be ascer- 

 tained. Both gradual and abrupt lengthening have induced unseason- 

 able maturation in fishes, but Baggerman (1957) demonstrated con- 

 clusively that abrupt increase is far more effective than gradual in- 

 crease and induces a significantly earlier onset of maturity, at least in 

 Gasterosteus. Two unusual instances of effective photoperiod altera- 

 tion are worth mention. When transported from the Southern to the 

 Northern Hemisphere, the South American fish Jenynsia lineata pro- 

 duced two clusters of broods within a single year, one during the 

 Argentine spring-summer before it was transported and one during 

 the Illinois spring-summer after it was transported (Turner, 1957), 

 thus duplicating results of forced emigration in animals of other 

 vertebrate classics (Bullough, 1951). That an anomalous second 

 spawning period may sometimes occur in the wild out of season is 

 indicated by the discovery (Lagler and Hubbs, 1943) of evidently 

 fall-spawned young of the normally spring-spawning mud pickerel, 

 Esox vermicidatus, in mid-November of an autumn so warm that 

 spring-flowering plants such as Forsythia blossomed. 



If it is stipulated that day length is here the paramount extrinsic 

 factor governing secretion of gonadotropin by the pituitary, a suprali- 

 minal photoperiod is one that induces a daily sum of pituitary activity 

 sufficient to maintain an effective blood level of gonadotropin. It 

 follows that a daily light ration of adequate duration might be equally 

 effective whether administered each day as a single long exposure or 

 as a series of shorter ones. Rhythmic secretion-restitution fluctuations 

 in the gonadotropic zone of the adenohypophysis of Rhodeus are re- 

 ported to have a restitution phase of some duration (Bretschneider and 

 Duyvene de Wit, 1947). This entails the possibility that a subliminal 

 undisrupted photoperiod if administered in an evenly spaced series of 

 shorter intervals of light adding up to the same number of hours of 



