PHOTOPERIODISM IN FISHES 661 



temperature ("experimental conditions"), often followed by simultane- 

 ous exposure of these variously treated groups to one and the same 

 condition ("test condition"). In other experiments, fish were exposed 

 to identical conditions but at different times of year, and subsequently 

 half the fish of each group were exposed to one, and the other half, to 

 the other of the same two contrasting test conditions. The evidentiary 

 test (result capable of sensory verification) was always terminal: (1) 

 maturation as manifest in nest building by males, oviposition by fe- 

 males, (2) its significantly prolonged suppression, (3) the time elaps- 

 ing between onset of a pertinent stage in the experimental-test condi- 

 tions and maturation. It was concluded that these fish may be in one 

 of four physiological (possibly gonadal) phases, the first (Table I) 



Table I. Physiological (Gonadal ?) Phases of the Three-spined Stickleback, Gas- 

 terosteus aculeatus. Operationally defined by their inferred responses (positive or 

 negative) to each of the four combinations of high or low temperature and long or 

 short days; a positive response means progression to the next or to any higher phase. 



(Adapted from Baggerman, 1957.) 



High Temperature Low Temperature 



Phase Long Days Short Days Long Days Short Days 



- + -H +(?) 



la -f - - + 



1 



lb + - + - 



2 -f + + + 



confined to immature under-yearling fish, the other three successively 

 traversed by adults reaching functional maturity. Each inferred physio- 

 logical phase is operationally defined solely in terms of a characteristic 

 pattern of positive or negative responses by fish presumed to be in 

 that phase to each of four combinations of long or short and high or 

 low temperature (Table I). By response is meant progression to the 

 next or through to any higher phase. There is a difference between 

 response and test, the former usually being subterminal. The two 

 might be expected to converge only when a day length-temperature 

 combination elicits a response in fish in the penultimate phase or 

 causes a progression to full maturity through all phases above the one 

 in question. Since the only evidentiary test is maturity as manifested 



