654 PHOTOPERIODISM IN VERTEBRATES 



January 1 were continued on normal days after the end of the experi- 

 ment. All these fish reached maturity SVz months later, within the 

 normal spawning period. 



The middle panel of Fig. 2 shows the effects in terms of maximal 

 egg diameters of long versus short days on the maturity of the bridled 

 shiners in the final experiment (Fig. 1). Egg numbers are shown on 

 the vertical, Qgg diameters on the horizontal, with the larger diameters 

 to the right. By December 23, contrasting effects of long versus short 

 days are obvious. Egg measurements were made a week before and 

 three days after January 1, when the first spawning occurred. Within 

 this interval, the eggs of long-day fish advanced from penultimate to 

 final maturity. Among short-day fish, there was a continuous slow in- 

 crease in the minimum, mean, and modal diameters of the largest 50 

 eggs from each ovary, but after the first month maximum diameters 

 were arrested at about 336 microns. In the absence of long days, egg 

 maturation this time of year, as in the European minnow, Phoxinus 

 phoxinus (Bullough, 1939 and Fig. 2), fails to go beyond an early 

 stage of the secondary growth phase (period of yolk accumulation). 

 The distributional spread of the largest 50 egg diameters decreases as 

 more and more diameters approach the critical diameter (336 

 microns). No eggs of the long-day fish had exceeded this critical 

 diameter by October 26, so that it cannot be inferred that the mecha- 

 nism productive of later phases of the secondary growth period, i.e., of 

 final maturation, had as yet been set in motion by the stimulus of long 

 days so effective later. This egg-diameter distribution, however, is 

 unique among all the others in being skewed. Its maximum is the 

 same as in all short -day fish, but its mean is less than that of the same 

 date in short-day fish, and its mode equals that reached only by Janu- 

 ary among short-day fish. Therefore, although the internal mechanism 

 governing egg growth up to the critical diameter does not require long 

 days for its operation, as shown by the ovaries of short-day fish, it can 

 be speeded up by a long photoperiod. 



Two different and successive internal mechanisms must therefore 

 be recognized. The first, in operation at least since September and not 

 requiring long days for its performance, governs egg diameter increase 

 up to a critical maximum (336 microns). It responds, however, to a 

 gratuitous long photoperiod with an accelerated increase in the num- 



