656 PHOTOPERIODISM IN VERTEBRATES 



bers of egg diameters reaching this maximum. The second mechanism, 

 governing the final stages of maturation and essential for increase in 

 egg diameters in excess of 336 microns, requires long days for its per- 

 formance, at least out of season, but it is refractory to this stimulus 

 before mid-November. The two sexes reached maturity simultaneously. 

 The testes of short-day males remained quiescent, with stages up to 

 primary spermatocytes. Those of the long-day males were the same 

 until mid-November when spermiogenesis began. 



In all experiments on Notropis the temperature was constant at a 

 level near the average of the natural breeding season, so that for effects 

 of various day length-temperature combinations on the cyprinid cycle 

 one must turn to the experiments on Phoxinus (Fig. 2 and Bullough, 

 1939, 1940). As in Notropis, full maturity in either sex required long 

 days combined with high temperature. Without this combination the 

 early stages of the secondary growth phase were not surpassed in the 

 ovaries, and primary spermatocytes were the most advanced stages 

 reached in the testes. With short days, high temperature is inhibitory 

 to Phoxinus, arresting egg development (Fig. 2), and although un- 

 doubtedly inhibitory to Notropis also, it permits a slow, steady increase 

 in tgg diameters (Fig. 2). Inhibition of egg development by high 

 temperature in the absence of long days must be a widespread and im- 

 portant adjustment mechanism among fishes, for a similar effect is 

 found in Rhodeus (Verhoeven and van Oordt, 1955), in Enneacan- 

 thus (Fig. 2 and Harrington, 1956), in Apeltes (Merriman and 

 Schedl, 1 94 1 ) , and is consistent with the reaction of Gasterosteus to 

 the same set of conditions (Baggerman, 1957). In all these forms, 

 moreover, the inhibition by high temperature seems to be overridden 

 by a long photoperiod. From the comparison of maximal egg diame- 

 ters in Phoxinus, Notropis, and Enneacanthus under different condi- 

 tions (Fig. 2), it appears likely that egg development in the absence 

 of long days proceeds to a critical diameter at a rate inversely propor- 

 tional to the temperature. At low temperature, judging from Phoxinus 

 (Fig. 2), the rate of progress to the critical diameter is unaffected by 

 day length, and gametogenesis does not proceed beyond the early stages 

 in either sex (Bullough, 1939). Additional experiments on the inter- 

 action of day length and temperature on the cyprinid cycle at different 

 seasons should prove rewarding. However, the failure of the same 



